Note: Descriptions are shown in the official language in which they were submitted.
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BACK~RO~ND
....
Calcium sulfate is known to exist in several
different chemical forms generally differing in gross chemical
composition only by the amounts of hydrogen and o~ygen
expressed as water (i.e in the same 2 to 1 ratio as in water)
relative to the amount of calcium sulfate expressed as the
simple salt, CaSO4. In this way, the art recognizes at least;
a hydrate (CaSO~.2H2O); a hemihydrate (CaSO~1/2 H2O); and,
.an anhydrite (CaSO4~. Additionally, the anhydrite and the
hemi-hydrate exist in at least two different forms, based on
different degrees of water solubility In addition to the
several different chemical forms that have been proposed for
. calcium sulfate, it is known to be polymorphous as well, having
at least two distinct crystalline forms: rhombic and
orthorneubic or mono-clinic, Other crystalline variations
including acicular, needle-like and whisker crystals have also
been recognized Reference to such variations in physical
form, as ribbon or tape-like crystals, columnar and rod-like,
. twinned and swallow-tailed, curved and prismatic, are
frequently encountered in di.scussions of calcium sulfate
or gypsum as the hydrated form is commonly known 7 Thus, it
appears that authors disagree as to the exact chemical
-composition and the physical form of the various transformation
products of gypsum The numerous attempts to define the
various forms of the product and its transformation mechanism
have not remoyed the uncertainty as to an understanding of
the composition, structure, properties, and behavior of
calci~ sulfate, nor provided a generally accepted correlation
between them,
There, nevertheless, have been numerous attempts
to explain the behavior of calcium sulfate either by
transformation of physical forms or by the addition or loss
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of water of hyd.ration; see for example, The Chemi.stry of
Cement and Concrete by Lea and Desch (Longmans; Green &
_ .
Co., New York, 1935), pages 17 to 20; Journal of The So _ tX
of The Chemical Industry No. 13 Vol, XXVI (July 15, 1907),
. _, . _~....
pages 727-738; U, S, Patent No, 3,59~,123; U. S, Patent No,
1,901,051, Elsewhere, such as J. Appl. Chem. 1968, pages
307 to 312, differences in various forms of calcium sulfate
formed in autoclaving are attributed to differences in such
` things as purity, particle size and thermal history of the
: 10 gypsum startlng materials~
The commonly used crystalline form of calcium
. sulfate known as plaster has long been an impor-tant product
of the rehydration of dehydrated gypsum stored as a powdered
heml-hydrate or anhydrite which has the ability to set when
rehydrated by simply mixing with water. Plaster products,
which generally are stated to be the hemi-hydrate or anhydrite
form, are generally manufactured by dehydrating gypsum, One
commonly used process involves autoclaving ground, natural
gypsum in an atmosphere of steam, The commercial methods
generally seek to obtain a powdered produc-t of uniform
particle size with indiYidual particles of minimum surface
area; though there are frequent references ~o the formation
of undesirable acicular or needle-like crystals, Early
references to needle~like crystals formed in producing
Plaster of Paris can be found for-example in U.S, Patent Nos,
757,649 and 782,.321~ Manufacturers sought to eliminate
acicular crystals either by avoiding the conditions of their
formation as in U,S, Patent No~ 3,~10,655, or in breaking
them up by pulveriæing,
Only within about the last ten years has it been
recognized that calcium sulfate whisker crystals could be
ad~antageously employed as reinforcement fiber and that
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conditions under which acicular crystals form might be
controlled to give high yield conversion of gypsum to whisker
~ibers having length to diameter ratio of at least 6 to 1 and
as high as 100 to 1 or higher, Whisker fibers of this type
have been produced for example as described by Eberl, e-t al
in U.S, Patent Nos, 3,822,340 and 3,961,105,
While these patents recognize a variety of uses for
. such fibrous calcium sulfate product, particularly as a
substitute for asbestos fibers, none of the products made in
accordance with these methods has actually achieved commercial
use, The lack of commerical acceptance i5 believed to be
largely due to the difficulty in dispersing and effectively
employing the fibrous product obtained as a tangled mass of
fibers that cannot be readily separated, When reinforcement
is attempted with such fibers, they remain in the form of
tangled lumps or balls and offer little practical benefit,
. Accordingly, a high strength calcium sulfate fiber
that is readily dispersible in liquid media is highly desirable
and it is an object of this invention to proYide an improved
20 method for the production of calcium sulfate whisker crystals
which can be readily dispersed for use as reinforcing fiber.
A further object of this invention is to provide a method by
w~ich ~ypsum is converted into stable whisker crystals with
average fiber length of 100 to 300 microns having strength
and aspect ratio conducive to greater usefulness as
reinforcement, It is a further object of this invention to
provide a method wherein substantially all of the gypsum is
converted to whisker crystals 50 microns long or longer and
aspect ratio of 10 to 1 or greater which crystals are obtained
as substantially discrete fibers easily separated and capable
of being uniformly dispersed in liquid media using conventional
techniques, These and other objects of the invention will be
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more fully understood from the description and examples
which follow
`DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention, it has
been found that a calcium sulfate whisker fiber product of
markedly improved dispersibility can be made from gypsum by
the autoc].ave method if the temperature and pressure are
maintained substantially constant at or just above the minimum
fiber forming condi.tions during substantially the entire
perioa of fiber foxmation The improved process also gives
longer fibers and more'complete conversion of the calcium
sulfate to fibers Further', the presence of small amounts
of dispersing agent such as tannins or tannic acid during
the fiber formation step eliminates the clumping or balling
generally encountered during the formation of calcium sulfate
whisker fibers especially when the average length exceeds
about 100 microns r
. As used herein, the term "calcium sulfate whisker
fiber" is intended to mean any filamentary form of calcium
sulfate including single crystals that haYe grown in
filamentary form and which can be composed of any of the
hydrated, hemi-hydrated, anhydrous or other calcium sulfate
compositions or comhinations thereof Whereas, the whisker
fibers made in accordance-with this invention may be
individual fibers of h.eterogenous compositions or different
fibers may have different compositions, it is belieyed that
generally the whisker fibers are formed as single crystals of
- calcium sulfate hemi-hydrate which upon calcining are
converted to crystals of the anhydrite
As alrèady discussed~ calcium sulfate is known to
exist in several differént chemical combinations with water
and one or more of these chemical forms can exis-t in several
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physical forms. The combinations and permutations of
chemical and physical forms is substantial and except when
otherwise specified herein, the term calcium sulfate is
intended to cover all of the various forms Where a
particular chemical or physical form is specified, it will be
understood that the calcium sulfate composition intended
is predominantly in that form though part of the composition
may be present as one or more of the transformation products.
As used herein, the term calcine is intended to
mean heating calcium sulfate to a temperature in excess of
about 500C and covers elimination of water and volati]es
ordinarily removed at those temperatures as well as any
transformations in the product which may occur during such
heat treatment
The term, fiber~ depending on the context, is used
- alternatively to mean an individual filament of one or more
calcium sulfate whisker crystals or collectively, the fibrous
mass of calcium sulfate filaments
The fibrous calcium sulfate produced by the method
of this invention is composed of discrete fibers having an
average length of about 100 to 500 microns and a diameter or
average cross section of about 2 to 10 microns~ with an
overall average aspect ratio ~length to diameter3 in the range
of about 50:1 to 100:1, and is substantially free of non-
fibrous calcium sulfate In referring to diameter of the
fiber and to aspect ratios, it will be understood that the
fibers may have varied cross-sectional configurations. As
used herein the terms diameter or cross-sectional dimensions
are intended to mean the average of the characteristic
dimensions of the cross section
Any commercially available gypsum, either naturally
occuring mined gypsum or gypsum produced as a by-product of
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industrial waste treatments, such as limed spent sulfuric
; acid wastes, can be used as starting material, Preferably,
the gypsum used will have a calcium sulfate content of 95%
; or greater and will be substantially free of organic
impurities,
As a general matter, the presence of unknown
impurit.ies should be avoided either as an ingredient in the
gypsum or in the water used in making up the gypsum slurry,
since impurities, particularly some organic impurities, can
materially affect the formation and quality of whisker
crystals, In most instances, however, particularly in the
case of dissolved inorganic salts, the effect will be to
shift the minimum fiber forming temperature, as described
elsewhere herein, which can be determined emplrically and
the process conditions adjusted accordingly, Gypsum with
impurities in almost any amount can be used, if the impurity
does not adversely affect the fiber formation and provided
it can be tolerated as a diluent, The gypsum used in the
preparation of the fiber described herein f was mined gypsum
containing 96~ plus of calcium sulfate dihydrate~ Distilled
water was used to make up the slurry though generally
a~ailable water can be used without requiring any significant
change in ~he method, As dispersing agent, there was used
a reagent grade tannic acid supplied by Fisher Scientific
Company, Fisher Building, Pittsburgh~ Penna3 Such tannic
acids are used in an amount between about 1 and about 100 ppm,
desirably between about 5 and about 50 ppm and preferably
between about 10 and about 30 ppm of the total reaction
mixture on a w~igh-t basis, There can also be utilized as a
dispersing agent any of the available tannic acid or tannic
products including synthetic tannins, e,g, naphthalenic
syntans, and ~egetable extracts, such as, tea extract, A
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737~
variety of tannin products are available commerically. Any
of the generally available materials such as are disclosea,
for example in M Nierenstein, The Natural_Organic Tannins
~London, 1934) can be used. Other materials which function
as dispersing agents without inhibiting fiber formation can
also be employed Certain of the known crystal habit
modifiers have been found to assist in dispersibllity as well,
though in using such materials,-it will be appreciated that
usually the form of the product will be substantially altered
as well, and the choice may be determined by the characteristics
required for the fiber product. Ohter materials suitably
employed to improve dispersibility, are for example, the
polycarboxylic acids such as succinic acid or benzoic acid
derivatives such as anthranilic acid. When using a dispersing
agent which also modifies , the crystal structure, the
concentration of the dispersing agent can be chosen so as
to achieve the dispersing effect without significantly
altering the crystal structure~ The amount required to produce
~ dispersion will be considerably less than the amount used
for crystal habit modification.
Use of organic additives, particularly the poly-
carbo~ylic acids~ such as, succinic acid, have long been
kno~n in the preparation of Plaster of Paris where these
materials have been added as crystal modifiers or as agents
to retard setting or curing after the plaster has been gauged
Such use as crystal habit modifiers, crystallization
inhibitors, or retarding agents, are disclosed for example
by U. S~ Patent Nos. 2,460,267 and 2,448,218, British Patent
No 563,019, U. S. Patent Nos 3,520,708 and 2,044,942
These materials especially the preferred tannic acids or
tannins, have not, however, been previously used to prevent
c]umping or balling of whisker crystal fibers during formation
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The product fibers are obtained in readily separable form
arranged in generally parallel planes and their dispersibility
may be maintained during subsequent filtering, drying, and
calcining The fiber product can then be readily dispersed
in an organic or aqueous liquid media using ordinary mixing
: and dispersing techniques to provide a uniform dispersion of
reinforcing fiber throughout the product
In carrying out the process, a gypsum slurry is
prepared by mixing finely powdered gypsum with water in a
mixing tank heated either directly or by steam injection
The concentration of the slurry-should be between about 5%
and about 10% by weight of gypsum and preferably about 7% by
weight. Concentrations below 5% could be used but require
handling increased volumes Concentrations above 10% could
be used but there is a gradual decrease of average fiber
length as concentration is increased The dispersing agent
is added in an amoùnt between 0 0005% and about 0,5% by weight
of slurry/ The exact amount of tannic acid, tannin, synthetic
tannin, or other dispersing agent used will depend upon the
specific choice of dispersing agent In all cases, the amount
of dispersing.agent to be used is that amount which;is
sufficient to prevent clumping and balling without inhibiting
the nucleation and development of whisker crystal fibers
The entire mixture is maintained in suspension by
stirring while the temperature is raised to about 80~ to 90C
The hot slurry is then pumped into a vented autoclave capable
of being rotated, to provide mild agitation during the fiber
formation, and equipped with a heating mantel and temperature
control to enable the temperature of the a~ltoclave to be
elevated at a controlled rate With the autoclave vented,
the temperature is raised to abou-t 100C and the autoclave
is sealed. Thereafterj the pressure in the autoclave will
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7374
correspond to the autogenous pressure developed by heating
and, depending upon the exact composition of the slurry, it
will approximate the pressure of saturated steam at the
corresponding temperature The temperature is then raised
rapidly to about 112C, after which the temperature is raised
at a rate of about 0.1C to about 2 5C per minute, desirably
at about 0.5 to 1.5C per minute and preferably at about 1C
per minute, up to about 5C to 20C above the minimum fiber
forming temperature Generally, the temperature is raised
to an optimum reaction temperature between about 115C and
about 130C and held constant at that temperature for a
period of up to two hours, and preferably for about 5 minutes
to one hour, during which period all or substantially all of
the calcium sulfate is converted to the fiber form -
The autoclavin~ is preferably done at a temperature
no higher than about 10.C~ and preferably no more than about
6C~ above the minimum fiber forming tempera-ture, This is
generally sufficient to achie~e complete conversion to the
whisker form in about 1 hour. It has been found that the
rate of whisker formation increases as the temperature in the
autoclave is increased beyond a certain minimum or threshold
temperature below which whisker crystals do not form within
any reasonable period of time Such te~perature is referred
to herein as the minimum fiber forming temperature, As the
temperature is raised beyond the minimum fiber forming
temperature,`fiber is formed more rapidly though it has also
been found tha~ the longest fibers and the most complete
conversion of calcium sulfate to fibrous product is obtained
by allowing the fiber formation to proceed at the slowest
rate In prac-tice the temperature will be selected so as to
allow for substantially total conversion to fibers within a
reasonable time~ say about 1 hour. The m:inimum fiber forming
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temperature can be determined by trial and error for a
particular gypsum slurry dependiny on the quality of the
gypsum and water used for make-up. When a dispersing agent
is added, it generally causes the minimum fiber forming
temperature to rise by about 4 to 8C and the temperature
for the autoclave step should be adjusted accordingly The
calcium sulfate slurry is not stirred during the fiber
forming period, although, gentle agitation is desired. A
sufficient degree of agitation to prevent sedimentation can
be easily accomplished by periodically inverting the autoclave
during the fiber forming stage. Other methods for achieving
gentle agitation such as use of a vertical gravity pipe
reactor or other continuous autoclaving methods such as
disclosed, for example, in U S Patent No 3,579,300 can
be employed.
Upon completion of the fiber formation, the loose
unfelted mass of fiber is separated from the excess water~
The separation can be readily accomplished by pressure
filtration without allowing the temperature to drop below
100C and preferably~ not below 105C.
The filter cake i5 immediately dried at a temperature
in excess of 200C and preferably, at about 400C The
dewatered product consists of easily separable free flowing
whisker crystal fibers that have a silky feel~ The reaction
product is consistently recovered as fiber in quantative
yield based on the amount of calcium sulfate in the starting
mixture The pressure filtration can be conveniently carried
out in a rotar~ pressure filter such as a BHS-FEST-Filter
which is a rotating drum type filter manufactured by BHS-Werk
Sonthofen, D-~g72, Sonthofen, West Germany Ordinarily, the
fibrous product can be fed directly from -the autoclave to the
filter using the autoclave pressure, with additional steam
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under about the 30 psig pressure, to maintain the product
under pressure during the entire removal and filtering
operation while the water content is reduced to less than
about 50% by weight of water in the filter cake As the
product is removed from the f ilter, the individual f ibers
appear layered in the f ilter cake or mat indicating little
three dimensional entangl.ement and it is desired to maintain
the f iber mass free of such entanglement during the subsequent
drying and calcining. In order to do so, the drying and
calcining are preferably carried out by transferring the fiber
mat, as removed from the filter, onto an endless conveyor belt
and passing it through an oven for drying and calcining.
Alternat.ively, the fiber mass can be dried and calcined by
other conventional means, It is, however~ preferred to
minimize the amount of -tumbling in order to maintain the
individual fibers relatively free of entanglement to permit
easy fiber dispersion when subsequently ~ixed with a resin
or cement.
Calcining is carried out at a temperature between
about 500C and about 650C/ preferably at about 600C, for
a period of less than 15 minutes and mostly for no longer
than.about 1 minute in order to avoid degradation of the
product~ The pH obtained with the final product immersed
in water should not exceed pH lO and preferably should be
less than pH 8 ~ower pH is obtained by lower calcining
temperatures and shorter calcining times
After removal from the calciner, the product is
allowed to cool in air. This product, which is believed to
consist of the insoluble anhydrite of calcium sulfate, will
remain stable for periods up to a year and longer; it will
not readily reconvert to hydrated forms in the presence of
moisture so that it can be used to reinforce aqueous matrix
~)rn: ft SJ. ~.,
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materials such as -the hydraulic cements~ Alternatively, if
the product is to be used in an organic resin matrix, without
a preliminary prolonged exposure to moist air, it can be used
without calcinlng simply removing it from the drying stage and
cooling The bulk density of the produc-t after calcining
is between about 10 and 20 lbs. per cubic foot, usually about
15 to 18 lbs ; the product can be readily compressed up to
about 30 lbs. per cubic foot without damaging the individual
fibers
The calcium sulfate fibers are sufficiently stable
after drying for use as reinforcement in non-aqueous systems
and after calcining for use in aqueous media However~ if
desired, the fiber can be coated as described by Eberl, et
al U S Patent No~ 3,961~105. If a coating is applied~ it
is necessary only to dry the product to a water content of
less than 1~ which is generally accomplished by drying at
300C to 400~C for about 5 to 15 minutes As described herein
the process is essentially a batch process, though it can be
made continuous by use of a continuous pressure reactor for
the autoclaving step In either batch or continuous~ it is
important that the handling, from the fiber forming stage to
complétion, be done with a minimum of mechanical agitation,
tumbling, shearing and the like~ which would cause the fiber
to clump or ball making it difficult to disperse uniformly in
the subsequent product
If desired, the water removed in filtering can be
recycled to the slurry stage with additional make-up water as
re~uired. In recycling a certain amount of the dispersing
agent is retained in the water and the amount added in make
up of additional slurry must be adjusted accordingly~
If desired~ coatings can also be applied to the
fibers to improve the wetability or the adhesion of the fibers
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2~3~4
to the plastic component when used as a reinforcing fiber
with~ for example, phenol formaldehyde, urea-formaldehyde~
melamine formaldehyde, polyurethane or other polymers The
use of such coatings is disclosed in Eberl, et al U. S.
Patent No. 3,9~1,105.
The calcium sulfate whisker fibers prepared by
the method of this invention as already stated are useful as
reinforcing agents for either organic or inorganic matrix
materials, particularly, the synthetic polymers. These fibers
; are especially suited for blending with organic or inorganic
matrix materials in liquid or in dry solid form.
Various methods of dispersion and blending of the
fiber in the plastic or other matrix materials can be employed,
although properties r especially mechanical properties~ of the
resultant composite materials may be affected by the choice
of dispersing means ~any varieties of commonly used
processing equipment, such as, single and twin screw extruders,
two-roll mills and Banbury mixers can be used to produce
excellent dispersions The-equipment must be specifically
run to produce good fiber dispersion in the plastic matrix
with a minimum amount of damage to the fiber
A two-roll mill can be advantageously used to
blend into small batches of thermoplastic materials. Usually~
two-roll mills are operated under atmospheric conditions
As with other types of processing equipment; the plastic
material should be fluxed on the mill before the addition
of fiber. It is recommended that the roll stock be removed
by a doctor blade or other means and then returned to the
mill. In order to attain recommended shear levels, it is
suggested that nip clearances greater than 25 mils be used
and that the roll speeds be in the range 100 to 125 r p m
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with a speed differential of 25 r,p.m, It is apparent that
the temperature at which the rolls are maintained is dictated
by the properties of the plastic material being worked, As
a guiding principle, roll temperatures should be maintained
at such a level that addition of the fiber to the plastic roll
stock causes the resultant roll stock to lose tackiness and
thus become easily workable.
A Banbury mixer can also be employed, ~s in the
case of the two-roll mill discussed above, it is very important
that the plastic material be in a condition of flux before
addition of the fiber, This is best accomplished by feeding
the plastic into a hea~ed hopper or holding bin and then
holding it under force in the mixing area until fluxing is
observed. Upon completion of fluxing the fiber may be added,
The blending should then be continued for a period of time
sufficient to achieve the desired distribution of fibers,
It is necessary to control the viscosity of the fluxed resin~
fiber composite. This can be done by controlling the melt
temperature by means of varying speed of rotation, cycle
time and the circulation through the rotors of either steam
or water as the situation requires. Generally speaking, it
is possible to increase the blending time so that improved
dispersion may be attained without degrading the fiber. For
example, a typical compounding-cycle for a thermoplastic
material would total three minutes, Twenty seconds would
allow fluxing to occur, and forty seconds would suffice to
add the fiber, Thus, two minutes are available for the
blending process. A temperature rise during the final -two
minutes of the cycle is commonly observed,
For extrusion, single or twin screw extruders can
be used to compound the fibers into resin compositions, The
temperature variation along the length of a twin screw
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extruder should be controlled so that fiber degradation is
kept to a minimum, Generally speaking, with higher melting
polymers, such as nylon, the temperature in the feed section
should be maintained about 25 to 50C above the required
melt temperature for the polymer involved, In the case of
low melting polymers such as polyolefins, it is suggested that
the feed section be maintained 10 to 30C above melt
temperature, The temperature profile in -the barrel will
depend on the speed of rotation of the screws since this
dictates the time the solid resin remains in the feed section,
Thus, larger temperature differences between feed and
completion sections are required at higher screw ro-tations,
It is essential to recognize that degradation of
fiber during the extrusion process may occur before the
fluxing of the resin, ~ccordingly~ configuration of the
screws should be such that the fiber and resin mix are gently
conveyed until softening of the resin occurs by the transfer
of sufficient heat~ Once fluxing has taken place, much
compressive and dispersive working can be accomplished with
little or no degradation of the fiber, Reduction of
compression in the feed section can be accomplished by using
deeper channels here than in the remainder of ~he screw, -
Good engineering practice would suggest a gradual reduction
in screw pitch following the feed section to avoid a situation
of immediate compression of the material, It is desirable
that the screw design incorporate kneading blocks since these
are known to aid in dispersion of the fiber. In accordance
with the infor~ation given above, these kneading blocks
should be placed in a section where the polymer has already
melted so that unnecessary mechanical working is avoided.
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In cJeneral~ standard desi~3ns and configurations
for dies and adaptcrs are suitable for use in fiber
extrusion and those configurations commonly used for PVC,
work well,
Combinations of calcium sulfate fiber and other
reinforcing fiber, such as chopped fiberglass, can be used
for example in polyester, or the calcium sulfate fiber can
be used to replace fiberglass, Cast composite specimens of
~ polyester resin and calcium sulfate fiber containing 5,5% by
volume or approximately 13% by weight of fiber are prepared
by dispersing the fiber in a polyester resin stirring and
cuxing at room temperature using 0,75% by weight methyl-ethyl-
ketone peroxide as a curing agent, ~esidual air is removed
from the mix under a vacuum of about two millimeters mercury
for a time period of about 5 minutes.
Irhe fiber is dispersed easily and gives a pourable
mixture of fairly low viscosity, The composite mix is then
cast into a block with dimensions approximately one quarter
inch thick by three inches wide by six inches long, The
test blocks are allowed to set at room temperature and are
then post cured at 110C for 30 minutes, The mechanical
properties of these polyester/fiber composites compare well
with corresponding asbestos and iberglass reinforced
composites,
Calcium sulfate fiber can be bonded with magnesium
~xysulfate (Sorél Cementl, By way of example, a mixture
composed of: calcium sulfate fiber, 38,7 wt, %; magnesium
oxide, 13.6 wt. ~; magnesium sulfate, 4.8 wt, ~; Natrosel*
250, 1.0 wt. %; Duponol*ME (dry)~ 0,3 wt. %; and water~
~1.6 wt, ~, is prepared by adding the water all at once and
using a high speed mixer to foam the mix. The mix, which
appears to be dry at first, becomes a heavy foamy mass with
* T~de ~arks
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continued mixing and is suitable for use as a foam-in-
place insulation.
The whisker fibers can also be used in place of or
in combination with asbestos in other applications such as
for reinforcement in cas-t magnesia cement compositions and
the like.
Particularly useful products prepared by using
the calcium sulfate fiber prepared in accordance with this
invention, are the formulations for friction elements such
as brakes, clutches, transmission bands, etc. wherein the
calcium sulfate fiber is used to replace part or all of the
asbestos fiber presently employed in such formulations.
Formulations for friction elements generally utilize phenolic
resins of the pulverized noyolak type. These resins are
utilized primarily in dry mix processes for the production
of disc pads, linings and truck blocks. A suitable
formulation is prepared by blending about 35 to 75 parts
preferably about 60 parts by weight of calcium sulfate fiber
prepared in accordance with the process of this invention,
with about 10 to 15 parts preferably about 13 parts by
weight of barytes, about 5 to 10 parts preferably about
7 parts by weight of Cardolite brand epoxy resin flexibilizer
made by the 3M Company of Minnesota, and about 15 to 25
parts preferably about 20 parts by weight of a phenolic
resin of the novolak type. If desired the fiber ingredient
can be used as a blend, suitably at a ratio between about 1
to 1 and about ~ to 1 of calcium sulfate fiber to asbestos.
The blended formulation is compression molded at about 200C
and post cured for about ~ hours at about 230~C. The cured
friction element when evaluated by various fric-tion -testing
procedures, which are standardized in the industry show the
calcium sulfate Eiber to be an acceptable reinEorcing material, particu-
larly, for light duty applications and as a diluent for asbestos. The
drn~ 17-
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calcium sulfate fiber is especially beneficial for improved
wear performance.
The following examples are given by way of
illustrating the improved gypsum transformatiorl process for
producing an improved dispersible calcium sulfate iber product.
Example 1.
Twenty-two grams of a commercial gypsum (Terra Alba)
is slurried in 200 cc of water (a concentration of 10% by
weight of total slurry). The slurry is heated to a temperature
of about 80C and pumped into an autoclave equipped with an
electric heating mantel and Variac control, With the autoclave
vented, the temperature is raised to about 100C and maintained
; until substantially all air is removed. The vent is then
sealed and the temperature raised to about 116C for about
15 minutes after which, the temperature is raised to about
126C and held for an additional 40 minutes at which time the
transformation to whisker crystals is complete. The slurry
is maintained in suspension throughout by mild stirring or
rotating the autoclave periodically. Excess water is removed
and the remaining fibrous mass is dewatered. The excess of
supernatent water is first blown out of the autoclave without
cooling using the pressure from the autoclave. A sample of
the fibrous product is r~moved and put into ethylene gl~col
to prevent rehydration Other organic water immiscible
solvents such as acetone, ethanol, etc could be used The
solvent is removed under vacuum and the sample fibers are
examined under the microscope. The product is composed of
whisker crystals having an overall average length of about
200 microns and cross sectional dimensions of about 2 to 3
microns. The remaining product while maintained at a
temperature over 100~C is filtered under pressure, the filter
cake is transferred immediately to an oven where it is dried
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to about 1% moisture content at about 400C for about 15
minutesO The dried fiber is then fed to a calcining oven
and rapidly heated to about 600C and held at that temperature
for no longer than about 0,5 to 1~5 minutes, After cooling
the product remains unchanged indefinitely.
Exampl
The experiment is carried out as in Example 1 except
that before autoclaving, tannic acid (Fisher Chemical) in an
amount equal to 0~0025% by weight of the total, is added to
the gypsum slurry. After venting and sealing the autoclave~
the slurry is heated to 116C and held for 15 minutes The
mixture is then heated to 122C and held for an additional 20
minutes after which, the partially reacted product is heated
to 128C for an additional 20 minutes. The sample fibers
examined under the microscope are between 200 and 300 microns
long with some even longer and an average diameter of 4 to
5 microns~ The reaction mixture from the autoclave has a
low ~iscosity and is easily pourable, The filter cake is
similar in appearance to that of Example 1 but with little
~r no th~ee dimensional int~ungling of fibers and a layered appearance.
Example 3,
This experiment is carried out as in Example 2
except that the reaction is stopped at 124C after about 20
minutes and partial fiber formation, The product is cooled
and reheated to 128C for about 20 minutes The sample
fibers are flat ribbon-like crystals, 200 to 300 microns
long and longer with cross sectional dimensions of about 5
to 10 microns The filtered product is similar in appearance
to that obtained in the preceding example,
Example 4.
.,
A solution of succinic acid is prepared by
dissolving 0 0712 grams of succinic acid in 300 cc of water.
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Experimental runs are carried out as in Example 1 using a
7% by weight gypsum slurry. The autoclaving is done at 130C
for 45 minutes. Three separate autoclave runs with slurries
containing, respectively 1 cc, 2 cc, and ~ cc of th succinic
acid solution are carried out~ The sample fibers removed
from each product and examined under the microscope were
rod-like crystals having the following average dimensions:
with 1 cc succinic acid ---- length 150 microns
diameter 2 microns
with 2 cc succinic acid ---- length 150 microns
diameter 2 microns
with 4 cc succinic acid ---- length 80-100 microns
diameter 5-7 microns
The filtered and dried products are similar in
appearance to those of preceding examples with little three
dimensional mixing,
Example 5.
.
This experiment is carried out as in Example 4
by adding 0.15% by weight anthranilic acid to the slurry
instead of succinic acid, The sample fibers had an average
length of 150 microns and an average diameter of 2 to 3
microns and the filtered and dried fiber product is easily
separated into discrete fibers similar to that obtained in
the previous experiments~
Example 6,
This experiment is carried out similar to the
procedure of Example 5 using a 7% by weight gypsum slurry
to which is added 0.007% by weight succinic acid and 0,15%
by weight anthranilic acid, The autoclaving is carried out
for 35 minutes at temperatures between 122 and 13~C, The
: sample fiber had an average whisker crystal length of 150
microns and an average diarneter of 5 to 8 microns,
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Example 7 ~ ~
A group of experiments is run using a 250 milliliter
Parr Bomb reactor and a stirrer running at high speed until
the temperature is nearly at the min.imum fiber forming
temperature tca 110C) and thereafter~ the gypsum is maintained
in suspension by rotating the reactor a full 360 about every
two minutes while the temperature is raised at a controlled
rate to 140C, The controlled rates of temperature increase
are 2,5C/min~ 1C/min " and 0,5C/min " respectively, In
each instance, the conversion goes substantially to completion.
Examples of the fibers produced in each case have the
following composition based on total number of fibers in the
sample,
@ 2,5C/min,
Length: 0 to 50 microns 5 - .7%
50 to 100 microns 28 - 30%
100 to 200 microns 50 - 52%
. 200 to 300 microns 14 - 16~
! @ 1. 0C/min.
Length: 0 to 50 microns 4 - 5%
.50 to 100 microns 8 - 10%
100 to 200.microns 27 - 30%
. 200 to 300 microns 28 - 30%
300 to 400 microns 14 - 16%
400 to:500 microns 10 - 12%
@ 0,5C/min, ..
Length: 0 to 50 microns 4 - 5%
50 to 100 microns 4 - 5%
100 to 200 microns 16 - 18%
200 to 300 microns 20 - 30%
300 to 400 microns 22 - 24%
400 to 500 microns 24 - 26%
A similar experiment is carried out with 0~0125%
by weight tannic acid added to the slurry, I'he product formed
has lower viscosity, is easily poured, has about the same
distribution on length o~ fibers, a somewhat larger average
diameter and is considerably more easily dispersed,
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Example 8
In this experiment a fiber sample prepared
according to Example 2 followed by drying at 400C and
calcining at 600~C Eor 1 minute is tested to determine its
reinforcing characteristics in commonly used engineering
thermoplastics. Separate samples for injection molding were
prepared usi.ng nylon and polyester. Pelletized resins of
Nylon 6,6 (Zytel 101 available from DuPont Corporation,
Wilmington, Delaware) and polyester (Valox 310, available from
General Electric Corporation, Pittsfield, Massachusetts) are
used as the resin~ The nylon and polyester pellets are yround
separately to about 50 mesh~ and each is blended with 50% by
weight of calcium sulfate fiber prepared as above to provide
injection molding samples of reinEorced nylon and reinforced
polyester, Substantially no clumping and balling occurs
: during blending. In each case, the blend is dried for 2 hours
j at 100C, The resin/fiber pellet is in~ection molded in a
: five-ounce Van Dorn hydraulic press capable of 75-ton clamp
pressure according to the conditions below:
. 20- Temperatures (~C? Mylon Polyester
:- rear cylinder 300 238
; front cylinder 313 241
. nozzle 316 243
: . mold . 135 66
melt 316 243
back pressure (psi)50 50
injection pressure (psi)
stage I 1500 1500
: stage II 1300 1300
injection speed (sec)
stage I
stage II 1.5 2.5
mold case tsec) 30 30
injection forward (sec) 4~0 4.0
screw speed (rpm) 70 70
total cycle time (sec) 34 34
The resulting reinforced molded plastic is
subjected to the following tests:
* Trademark
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Tests
.....
Flexural: Strengths and moduli are measured
on a Baldwin-Tate-Emery Testing.
machine according to ~STM D-790.
Tensile: Strengths are obtained according
to ASTM D-638 on an Instron test-
ing machine,
Wet Tensile: Tests are conducted on specimens
exposed to 50C water for 24 hours,
Hea-t Distortion: Tests are performed with a Tinius
Olsen HDT~ bath using phenyl sili-
cone oil elevated at a rate of 2C
per.minute (ASTM D-648),
Izod Impact: Notched and unnotched tests are
performed according to ASTM D-256,
Modified Gardner A 1/2 lbo weight is dropped until
Impact Test: the bottom of a 4" x 2 1/2" x 1/8"
plaque exhibited a fracture,
Results
20 ~ Polyester
Flexural Strength~ psi containing 50 wt,% containing 50 wt,%
calcium sulfate calcium sulfate
fiber fiber
. Dry lg,000 14,000
Wet 9~000 11,000
Tensile Strength, psi. 11~000 8,000
Izod Impact
Notched 0~5 0 5
ft,lb,/in~
.30 Unnotched 4,0 4,0
Gardner Impact, in,lb, 3,5 3,5
Heat Deflection Temp.! F 450 350
Scanning electron microscope studies o~ fracture
surfaces of the calcium sulfate whisker fiber reinforced
nylon show good fiber distribution and a good degree of bonding
between matrix and reinforcement,
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Example 9
. . ~
Calcium sulfate fibers prepared by using a
dispersing agent in accordance with the methods of Examples
2 and 9 are prepared for evaluation as reinforcement for
PVC pipe using powdered compound with the fiber added in a
latter stage of the blending cycle. Pipe extruded from PVC
compound blended with 20 and 30 parts of calcium sulfate
fiber per hundred parts of resin show good impact and a high
modulus of elasticity~ The physical properties of the fiber
reinforced PVC pipe are shown below with comparison to
extruded P~C pipe using asbestos. PVC powder compound is used
for all pipe extrusionî the reinforcement being introduced
with the compound at the blender.
Physical Properties of Reinforced PVC
Càlcium Sulfate Fiber Asbestos
` Physical Property 20 pts. 30 pts, 20 pts,
" .
Modulus (Long)676~000786,000 550,000
Modulus (Cir.)585~000691~000 556,000
Tensile (Long)6,600 6,200 5,810
Tensile (Cir,)6,400 4,830 5,450
Impact 23C
Ave. ft,-lb./in. wall 1~050 - 185 195
This application is a division of copending
Canadian application Serial No. 314,239, filed October 25, 1978.
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