Note: Descriptions are shown in the official language in which they were submitted.
~S670
FIRE ~ESISTANT GYPSUM BOARD
CONTAINING CALCIUM SULFATE ANHYDRITE
Background of the Invention
Field of the Invention
This invention relates to improved gypsum formulations, and
more particularly to gypsum board formulations whereby increased
protection against fire is attained.
Gypsum board products, comprising a monolithic core of set
gypsum and a cover sheet (generally paper) encasement, are well
known in the art. They are widely used in the construction of
interior walls and ceilings and are variously termed gypsum panels,
plaster board, gypsum wallboard or the like.
The chemically combined water (about 21% by weight of the
gypsum) contributes to the effectiveness of products containing
it as a fire barrier in various building and construction products.
When gypsum board or set plaster formulations are exposed to fire,
the water is slowly released as steam, retarding heat transmission
for a time as the gypsum calcines. The heat resistive properties
of various gypsum building materials have been determined by
testing facilities on fire testing of assemblies performed in
accordance with the American Society for Testing and l~aterials
(ASTM) procedures. For example, ASTM C 36 Section 3.3 provides
a special fire retardant designation, type X, for gypsum wallboard
that provides at least one hour fire retardant rating for boards
5/8 inch (16 millimeters) thick, or 3/4 hour fire retardant rating
for boards 1/2 inch (13 mm) thick, when the boards are applied to
a test partition in single-layer nailed application on each face
of load bearing wood framing members and the assembly tested in
accordance with the requirements cf ASTM method E 119.
From studies of the actions of gypsum board when exposed
to a fire, such as in a laboratory fire test, it has been generally
evident that there is a substantial shrinkage of the board core
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at sustained high temperature with consequent cracking, which not
only contributes to passing excessive heat and hot gases through
the test wall but also hastens the disintegration of the board
under these adverse conditions. Also, as the gypsum calcines it
loses its inherent set strength.
Description of the Prior Art
To increase the fire resistant properties of these products
it has been conventional to introduce certain fibers and unexpanded
vermiculite ores into the slurry of calcium sulfate hemihydrate
~plaster or calcined gypsum or gypsum stucco) and water during the
board forming process. This concept is disclosed in U.S. patent
numbers 2,526,066; 2,681,861; 2,744,022; 2,803,575; 2,853,394;
3,454,456 and 3,616,173. These patents basically teach the use
of certain unexpanded vermiculite to offset the shrinkage of the
board core during the heat exposure, the unexpanded vermiculite
expanding as the chemically combined water present in the gypsum
is driven off. As this heating also tends to degrade the cohesive-
ness of the gypsum, reducing the strength and integrity of the core,
the fiber component of the core formulation imparts a mechanical
binding or matting effect to help hold the calcining gypsum together
and l~eep it from disintegrating and falling into the test furnace.
Further, U.S. patent 3,616,173 notes that particular pro-
portions of certain small inorganic particles will further improve
the overall fire resistant properties of the board cores containing
unexpanded vermiculite. Thus certain clays of less than 1 to about
40 micrometer (um) size and either colloidal silica or alumina of
less than 1 micrometer size, or mixtures thereof, are said to pro-
vide some fire resistant properties in further cooperation with
the ore fiber mixture. I).S. patent 3,454,456 indicates that
having some proportion of the unexpanded vermiculite present as
fine sized particles smaller than 100 mesh (147 um) helps to pre-
vent large surface fissures and spalling on the board core. This
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patent calls for the use of an unexpanded vermiculite of a par-
ticle size which will pass through 50 ~.S. Standard mesh t297 um)
and be retained upon a 140 mesh sieve (105 um) for accomplishing
low fire shrinkage and low spalling.
Summary of the Invention
The present invention was discovered upon retesting a board
core sample fire tested a day before, whereupon it was determined
that the sample did not shrink at all in the subsequent fire test,
but expanded to 0.150 inches (0.038 cm) in a one hour exposure.
Upon further evaluation, it was discovered that during the first
test the gypsum of the board core sample had been converted to
the anhydrite II form, and this led to the idea of using anhydrite
in the core composition. The anhydrite may be regarded as a pre-
heat-treated and pre-shrunk gypsum additive which provides improved
fire resistant properties.
It is an object of this invention to provide improved fire
resistant gypsum board and plaster formulations.
It is another object and advantage of this invention to
provide a fire resistant gypsum board core and other calcined
gypsum formulations that not only do not shrink as a result of
fire exposure but may expand while providing integrity to the
heated gypsum material.
The objects of this invention are accomplished basically by
incorporating in the gypsum formulation, instead of the customary
unexpanded vermiculite, about 2-40 weight % of calcium sulfate
anhydrite II, alone or in further combination with small amounts
of textile glass fiber or more substantial amounts of unexpanded
vermiculite or wollastonite.
Brief Description of the Drawings
Figure 1 is a graphic illustration of expansion/shrinkage
curves obtained during one hour small scale fire testing of a
control and two formulations of the present invention.
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Description of the Preferred Embodiments
The major ingredient of the gypsum composition of the
invention is set gypsum, i.e. calcium sulfate dihydrate. It is
formed by the hydration crystallization of calcined gypsum which
has been slurried with water along with conventional additives
according to the usual techniques. The calcined gypsum may be
either alpha or beta hemihydrate, soluble anhydrite, or mixtures
thereof, from natural or synthetic sources. Conventional additives
may be added in customary amounts to gypsum formulations to impart
desirable properties and to facilitate manufacturing, such as,
for example, foaming agents, accelerating agents, retarding agents,
dispersing agents, core adhesives, and mixtures thereof.
In the manufacture of gypsum wallboard, the core material
is generally made by metering the dry ingredients and water into
a mixer and therein generating a foam to control the density of
the resultant core material, such as by adding a dilute surface
active foaming material solution to the mixer in proportions
suitable to form a pourable aqueous slurry. The slurry is dis-
pensed through one or more outlets at the bottom of the mixer onto
a moving conveyor carrying a cover sheet, such as of a multi-ply
paper. Another paper cover sheet is then placed on top of the
slurry, so that the slurry is sandwiched between two moving cover
sheets which become the facings of the resultant gypsum board.
The thickness of the resultant board is controlled by a forming
roll, and the edges of the board are formed by appropriate mechani-
cal devices which continuously score, fold and glue the overlapping
edges of the paper. Additional guides maintain thickness and
width as the setting slurry travels on the moving belt. The board
panels are than cut, trimmed and passed to dryers to dry the set
but still somewhat wet boards.
~ he improved gypsum board of the present invention is
essentially a board core of set gypsum and calcium sulfate anhy-
drite II, i.e. the water insoluble form of calcium sulfate in
. . .
12;~5~70
contrast to the soluble form which hydrates quickly with the
moist air or water.
The core may also contain glass fibers for improved strength
and integrity, but it can be made without them. The core formula-
tion can include the anhydrite in addition to the customary amounts
of gypsum or the anhydrite may be in partial substitution for
customary amounts of the gypsum. The gypsum core may also include
additional inorganic mineral fillers, particularly of acicular
particle shape such as wollastonite (CaSiO3) or about 1-5% by
weight of unexpanded vermiculite.
It is preferred that a slowly calcined dead burned anhydrite
II, and most preferably of fibrous particle shape having an aspect
ratio of greater than 20:1 of length: diameter be used, but slowly
or rapidly calcined anhydrite II particles may be used. Dead
burned (calcined at greater than 1200C) anhydrite prepared from
natural mineral gypsum or gypsum synthesized from industrial pro-
cesses may be used as well as natural mineral anhydrite. Mixtures
and blends of different anhydrite II forms may be used. The anhy-
drite II may be present in an amount ranging from about 2-40~ or
more by weight when used by itself. It may also be present in an
amount of between about 1% and 10~ by weight particularly where
wollastonite, unexpanded vermiculite, and~or glass fiber fillers
are present.
The following specific examples will further illustrate
various specific embodiments of the compositions and products of
the present invention. All amounts are expressed as parts by
weight unless specified to tile contrary. Of course, lt is to be
understood that these examples are by way of illustration only
and are not to be construed as limitations upon the present invention.
EX~IPLE 1
In a first series of evaluations, several formulations were
formed into nominal 1.3 cm thick paper covered gypsum board panels
on a commercial manufacturing size gypsum board forming line.
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These boards were all made as nearly the same as possible, with
substantially identical amounts of set adjusting agents, consis-
tency red~cers, binding aids, foam, water and other adjuvants of
commercial gypsum boards. Aliquot panel portions measuring 15.25
cm by 25.5 cm were taken from the full si~e panels and submitted ~-
small scale fire testing.
For the small scale fire test, the board sample was placed
vertically in front of a test furnace composed of fire brick and
had a front plenum opening 0.635 cm larger than the board sample.
The furnace was equipped with natural gas burners arranged so
that the flames bathed the sample, rather than impinging upon
particular spots of the sample, and so that the temperatures along
the exposed face of the board sample were essentially uniform.
Temperatures within the furnace and on the exposed face of the
sample were measured by thermocouples. In addltlon, the sample
was restrained within the brackets of a spring strain gauge to
measure the expansion and contraction of the sample during the
one hour fire test. For each test the furnace temperature was
held as close as possible to the same time-temperature curve.
The furnace fire was started after the panel was set in place, and
the temperature raised from ambient to 538C-593C over the first
5 minutes of the test, to 746C-755C at 10 minutes into the test,
and maintained at about 755C for the remainder of the hour long test.
Normally, conventional gypsum board core formulations with-
out any particular fire resistant additives undergo a thermal
expansion of about 0.05-0.06 cm in the first 10 minutes of this
test as the paper cover sheets burn off, and then start to shrink
as the gypsum in the core calcines. The maximum shrinkage ordi-
narily takes place in the first 40 minutes of the test. In this
series of tests, the control was a currently preferred fire resistant
formulation according to the hereinbefore discussed patents con-
cerned with unexpanded vermiculite ore and chopped segments of
textile glass fiber. The anhydrite used in samples 2-5 was a
. _ . . _ , . _
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particulate dead burned calcium sulfate anhydrite ground to a
mean particle size of 1.5 micrometers. Integrity of the samples
was measured by visual observation of the sample during and at
the conclusion of the test. In general expansion of the vermicu-
lite caused microcracks and weakened the burnt core so it cannot
withstand it's own weight. With the anhydrite, smooth burned
high integrity cores without visible microcracking were obtained.
Representative results of the fire testing were as follows:
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In this example the formulation additions were by weight
based on the total composition of the board core ingredients. The
results of the control and samples 2 and 3 are graphically pre-
sented in FIGURE L. From the FIGURE it may be clearly seen that
the anhydrite sample 2 without any unexpanded vermiculite ore and
without any glass fiber provided less expansion/shrinkage deviation
during the 60 minute test than the standard fire resistant formu-
lation. In the FIGURE, sample 3 further shows that the amount of
anhydrite may be considerably decreased and fire resistance main-
tained with further benefit from including some unexpanded vermicu-
lite. Sample 4, not set forth graphically, had equivalent shrinkage
performance in the last half hour of the fire test as sample 3 but
it had less than half of the anhydrite additive. Sample 5 further
shows very good expansion/shrinkage control in an anhydrite formu-
lation with low amounts of the anhydrite and without any unexpanded
vermiculite.
EXAMPLE 2
In a second series of evaluations in accordance with the
fire test procedure of Example 1, portions of the hemihydrate used
in conventional gypsum board slurry formulations were replaced with
various anhydrite materials, and small size paper covered gypsum
panels were formed on a laboratory size gypsum board forming line.
The particulate anhydrite II materials used are identified in the
data. The FRANKLIN FIBER filler used was a calcium sulfate whisker
fiber in anhydrite II form having single crystal average diameters
of about 2 um and lengths typically of 50-60 um. For convenience,
only the cumulative shrinkage at the end of the one hour tests is
reported, with representative results as follows:
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Sample 1 hour % change in comparison to
shrinkage -cmControl A Control
Control A - no fire .589, with
resistive core large cracks
additives in core
Control B - 5%
vermiculite ore
& 0.3% glass flber .188
Sample ~8
8.3% reground board
core from prior
testing .188 68~ -
Sample #9
8.3% ground anhy-
drite rock & 0.3%
glass fiber .165 72% 12%
Sample #10
8.3% commercial
SNOW T~ITE~ anhy-
drite particulate
filler, average
8 micrometers
particle size &
0.3% glass fiber .165 72% 12%
Sample #11
30% slow burned
particulate anhy-
drite, average 9
micrometers particle
size ~ 0.3% glass
fiber .008 99% 96%
: Sample #12
23% particulate anhy-
drite filler, average
: particle size 2-2-1/2
micrometers, 5% ver-
miculite & 2.25% boric
acid +.188 expansion 132% 200%
Sample #13
16.7% particulate
anhydrite (2-2-1/2 um),
- 5% vermiculite, 2.25~
boric acid +.119 expansion 120% 164%
Sample #14
6.7% anhydrite
(2-2-1/2 um) 5%
vermiculite ore .056 91% 70%
PlFRANKLIN FIBER
calcium sulfate filler +.033 expansion106% 118%
Sample #16
3% FRANKLIN FI~ER
calcium sulfate filler .175 70% 7%
,~ Sa3mP3%e FlANKLIN FIEIER
calcium sulfate
filler & 3.3% woll- .099 83% 47%
astonite
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From these results it is quite clear that hlghly variable
amounts of different forms of anhydrite provide improved fire
resistant gypsum board core formulations. With sample B, ground
for this example to average particle size of 12 micrometers, and
sample 9, ground to average 8 micrometers, shrinkage characteristics
equivalent to the use of unexpanded vermiculite ore and glass fiber
was obtained. SampLes 12-14 show from equivalent to expansive
results with a finely gound anhydrite particular filler; and samples
15-17 show from less shrinkage to expansion with small amounts of
an anhydrite whisker fiber.
EXAMPLE 3
Full size nominal 1.3 cm thick paper covered gypsum panels
were made 1.2 m wide by 3.6 m long on commercial scale gypsum board
manufacturing equipment, and the panels submitted to large scale
fire testing.
For this fire test a full size non-load bearing wall assembly
was erected by screw attaching four panels onto 89 mm steel studs;
and the heat transmission from a test fire monitored by 9 thermo-
couples placed in accordance with ASTM E-ll9. This test evaluates
the times an assembly can endure a standard fire before (a~ the
average temperature readings of a-l tkermocouples attached to the
unexposed face of the panel rises 200 degrees Fahrenheit above
ambient temperature and (b) an individual thermocouple attached
to the unexposed face of the panel rises 125 degrees Fahrenheit
above ambient temperature.
A control formulation panel containing 5~ unexpanded vermicu-
lite ore and 0.3% textile glass fiber obtained a single point
unexposed side failure time at 203Cof 43 minutes and 40 seconds,
and an averaged multiple points unexposed side failure time at
162C of 44 minutes. Observation further showed a maximum exposed
side opening at a panel edge of 0.635 cm and signs of spalling and
hairline cracks on the exposed side.
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~ panel with a core formulation including 3.25% FRANK1IN
FIBER calcium sulfate deadburned anhydrite whisker fiber filler
and ~ 3.25% wollastonite as used in ~xample 2 obtained longer
exposure times of single point failure at 45 minutes and 30 seconds,
and multiple points failure at 46 minutes and 12 seconds. Obser-
vation showed maximum deflection of 1.270 cm, maximum opening of
0.159 and one large crack alongside one of the studs.
From the foregoing it is clear that the formulations of the
present invention are most useful in the formation of gypsum wall-
board cores whereby a markedly improved fire rating for systems
employing such wallboard may be obtained. Moreover the for~ulations
can be used in the formation of other products based upon a set
gypsum core obtained from the mixing of water with a dry calcined
gypsum formulation. Thus, dry calcined gypsum plaster mixes based
on the formulations of the present invention may be used, for
example, in metal casting plasters and in dry plaster mixes which
when added to water, can be applied to surfaces such as over steel
beams and girders or cast into partition blocks or ceiling tiles
and panels to give improved fire protection.
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