Note: Descriptions are shown in the official language in which they were submitted.
RD-15,370
BLE~DING TEMPE~ATURE SENSITIVE COMPONENTS INTO
A SILICONE MODIFIED THERMOPLASTIC
Bac~ground of the Invention
This invention relates to a method of producing
blends of thermoplastic polymers and high viscosity
silicone fluids which contain temperature sensitive
additives. More particularly, this invention relates to
a one-step process for producing a thermoplastic/
silicone fluid blend containing additives sensitive to
the blending temperature.
Blending -thermoplastic polymers with silicone
fluids often provides blends with desirable enyineering
properties and improved flame retardance, such as where
the silicone fluid is part of a flame retardant package.
Examples of such desirahle blends are disclosed by
McLaury, et al. in U.S. Patent 4,273,691, and by Frye
in U.S. Patent 4,387,176; both disclosures being
assigned to th~ same assigneeO
Producing thermoplastic/silicone fluid blends
with temperature sensitive additives has presented cer-
tain problems. The dispersion obtained when blending
high viscosity silicone fluids and certain thermoplastic
compositions has been found to be directly related to
the magnitude of the temperature during blending. The
limitation on the magnitude of the blending temperature
is typically the temperature at which the thermoplas-tic
polymers within the thermoplastic composition degrade.
For such blends there is often an optimum blending
RD-15,370
temperature range where a high degree of dispersion is
obtained with minimal polymer degradation. ~en blend-
ing at these optimum tempera~ures, blends with superior
engineering properties are obtained. Often a desirable
additive is sensitive to these optimum blending temper-
atures. An example of such an additive is the flame
retardant aluminum trihydrate. To avoid loss of the
additive's properties during blending, the blending
temperature must be reduced. As a result, the engin-
eering properties of the thermoplastic/silicone Eluid
blend suffer.
The present method of avoiding the loss of
superior engineering properties and maintaining the
integrity of the additive is to blend the high viscos-
ity silicone fluid and thermoplastic composition at an
optimum blending temperature in the absence of the tem-
perature sensitive additive and then subsequently blend
in the temperature sensitive additive at a lower temper-
ature. This procedure is disadvantaged in that two sep-
arate blending procedures are required. ~o produce
these thermoplastic/silicone fluid blends continuously,
two extruders are required, i.e., twice the equipment
required for conventional blending is utilized. Alter-
natively, the blend sample may be ~lended within one
extruder twice, once at the optimum blending tempera-
ture and once with the sensitive additive. Therefore,
either tbe rate of production suffers or the equipment
required must be increased when sensitive additives are
incorporated in the blend.
It is desirable to produce thermoplastic/sil-
icone 1uid blends containing additives sensitive to
the optimum blending temperatures in a continuous, one-
step procedure. The present invention is based on the
discovery that the blend may be cooled within the ex-
truder by feeding in a solid thermoplastic composition
RD 15,370
without significantly affec-ting the dispersion o~
blend constituents.
Summary of the 'Inven*ion
A method of continuously producing a thermo-
plastic/silicone fluid blend comprising the steps o~:
(a) Blending a high viscosity silicone fluid
having a viscosity of at least 90,000 centi-
poise at ambient temperature and a primary
thermoplastic composition comprising one
or more thermoplastic polymers within an
extruder at a blending temperature sufficiently
high to melt said thermoplastic composltion,
the weight ratio of the primary thermoplastic
composition to the h.igh viscosity silicone fluid
providing a value in the range of 100 to 1;
(b~ Feeding in a manner as to achieve some cooling
of the blend of step (a), one or more additives
sensitive to said blending temperature with a
secondary thermoplastic composition comprising
2Q one or more thermoplastic polymers into said
extruder downstream of the point where the
bl.ending in step (a) originates, -the weight
ratio of the secondary thermoplastic composition
and the blend of step (a) providing a value in
the range o~ 1 to 0.01; and
~c~ Blending said blend of step (a) with the
secondary thermoplastic composition and the
additives of step (b~.
Obj'e'c'ts o'f' the' I'n'ven't'ion
An object of the present invention is to provide
a simple, one~step, continuous process for producing
large quantities of thermoplastic/silicone fluid blends
containing additive sensitive to the blending
temperature.
~ ,~a^~
z~
RD~lS,370
Another object of the present invention is to
produce thermoplastic/silicone fluid blends having ad-
ditives sensitive to the blending temperature with im-
proved property profiles.
S ~nother object of the present invention is to
introduce additives sensitive to the blending tempera-
ture of thermoplastic/silicone fluid blends without af
fecting the dispersion of the high viscosity silicone
fluid.
Description of the Preferred Embodiments
_
The objects of the invention and other objects
are accomplished by blending a high viscosity silicone
fluid and a primary thermoplastic composition comprised
substantially of one or more thermoplastic polymers
within an extruder and feeding temperature sensitive ad-
ditive~ with a secondary thermoplastic composition com-
prising one or more thermoplastic polymers downstream of
the point where blending between the high viscosity sil-
icone fluid and the pximary thermoplastic composition
takes place. The additives and secondary composition
are then blended with the thermoplastic/silicone fluid
blend witllin the extruder.
In the process comprising this invention an
extruder is utili~ed to perform the blending steps.
The process does not require a particular extruder or
screw geometry to achieve the desired objects. ~ow-
e~er, a particular extruder ox screw geometry may be
preferred to provide suitable mixing and to avoid exces-
sive degradation of the thermoplastic polymers. Twin
screw extruders are often preferred for their high
shear rates, their distribution of shear rates, ~n~ the ty,oe
of agitation applied to the melt.
The blending of the high viscosity silicone
fluid and primary thermoplastic composition that occurs
.. . . . .. . . . ... . ... . . .. . . .... ... .. . . . . .. . . .
R~-15,370
within the extruder may be carried out in accordance
with the process disclosed U.S. Patent 4,446,090
.. ... . .
issued May 1, 1~84 to Lovgren et al wherein t~e primary
thermoplastic composition is fed into the feed hopper
and melted within an extruder, the high viscosity sili-
cone fluid is fed into the molten primary thermoplastic
composition within the extruder and said molten primary
thermoplastic composition and said high viscosity sil-
icone fluid are blended in the remaining portion of the
extruder.
The blending of the high viscosity silicone
fluid and the primary thermoplastic composition may also
be accomplished in accordance with more conventional
processes as disclosed in U.S. Patent 4,446,090,
~. . .
l~ issued May l, 1984, wherein ~:he sol~d primary thermo-
plastic compositlon and high viscosity silicone fluid
are premixed to provide a uniform feedstock suitable
for placement within the feed hopper of a conventional
extruder. The high viscosity silicone fluid and the
primary thermoplastic composition are then heated simul-
taneously to a temperature sufficiently high to provide
a viscosity for the thermoplastic polymers which allows
them to flow and be blended with the high viscosity
silicone fluid.
In both blending procedures, the blending
temperature is determined by the thermoplastic polymers
within the primary thermoplastic composition. The
blending temperature must be sufficiently high to melt
the pr~mary thermoplastic composition. Suitable blend-
ing temperatures for primary thermoplastic compositions
containing amorphous polymers typically fall within the
range having a minimum value of about 7 0 to 80 degrees
above the glass transition temperature of the polymer
and the maximum ~alue of about the temperature where
degradation result~. Where ~he primary thermoplastic
. . . .. .. .. . . , . , . . . , . , , , , ~ , . .. .. .
~lZ~
RD-15,370
composition contains crystalline polymers sultable tem-
pexatures fall within the range having a minimum value
of about 20 to 30 degrees above the crystalline meltin~
point of said polymers and a maximum value of about the
degradation temperature of said polymers.
The extent of dispersion of a high viscosity
silicone 1uid within the primary thermoplastic compo-
sition during blending is dependent on the processing
temperature at which blending takes place. Therefore,
when attempting to maximize the dispersion of blend
constituents in thermoplastic/silicone fluid blends,
the blending temperature is adjusted to provide maximum
dispersion. Often the degree of dispersion obtained in-
creases with temperature. In such a situation, the op-
timum blending temperature range may approach the
degradation temperature of the polymers within the
primary thermoplastic composition. However, even at
these t~mperatures maximum dispersion is ob-tained with
zero or minimal polymer degradation.
Once the primary thermoplastic composition and
high viscosity silicvne fluid are blended at a blending
temperature within the range defined above, the temper-
ature sensitive additives which are desired in t~e fin-
ished blend are introduced into the extruder with a
secondary thermoplastic composition comprising one or
more thermoplastic polymers. The quantity of secondary
thermoplastic polymer is preferably large enough to re-
duce the heat of the molten blend within the extruder to
a point where the temperature sensitive additives will
not be affected. By introducing solid thermoplastic
polymer into the hot blend, sensible heat of the blend
is consumed as the temperature o~ the solid thermoplas-
tic increases to the process temperature. Where the
solid ~hermoplastic polymer is crys~allin~, an addi~
tional amount of heat, the heat of fusion, which is
required to melt the polymer is also consumed.
~2~
RD-15,370
The temperature sensitive additives and the
secondary thermoplastic composition can be fed anywhere
along the extruder provided it is downstream of the
point where blending of the high viscosity silicone
fluid and primary thermoplastic composition initiates.
It is preferable to introduce the additives and second-
æy thermoplastic composition at a point on the extruder
where the desired dispersion has been obtained within
the blend. This point can vary depending on the ex-
trudex and on the blending process. For example,where the blending is achieved by a side feed process,
as diqclosed in U.S. Patent Number 4,446,090, issued
May l, 1984,'`adequate blending may not be obtained be-
fore ~he mid~point of the extruder, since no blending
takes place in the extruder until the silicone is in-
jected. ~here a twin screw extruder is utilized, it is
preferable to feed the additives and solid secondary
thermoplastic composition at a point about Z/3 the ex-
truder length froM the feed hopper. This ensures that
the secondary thermoplastic composition is dispersed
within the blend and that blending between the high
viscosity silicone ~luid and primary thermoplastic
composition is adequate.
The de~ree of agitation necessary to ~lend
the secondary thermoplastic composition into the molten
blend within the extruder is not as high as that re-
quired to initially disperse the high viscosity sili-
oone fluid into the primary thermoplastic composition.
~ere the secondary thermoplastic composition and the
primary thermoplastic composition are similar, the de-
gree of agitation that is necessary is even less.
The secondary thermoplastic composition may
be ~ed slmultaneously with the temperature sensitive
additives or it may be fed before the additives so as
to cool dow~ ~he blend within the extruder prior to
.. ... . . .. .. . .. .. . . . . .... . . ... ... . ..... . . .. . . ... . . .. .. .. . . . ..
~2629~% ~D-15,370
introduction of the additives. As indicated above, the
secondary thermoplastic composition mus-t be introduced
downstream of the point where blending between the
high viscosity silicone and primary thermoplastic com-
position is initiated. It is preferable to introducethe secondary thermoplastic composition subsequent to
obtaining the desi~ed dispersion of high viscosity
silicone within the primary thermoplastic composition.
Once the secondary thermoplastic composition
and temperature sensitive additives are introduced into
the extruder, they are blended with the thermoplastic/
silicone fluid blend within the extruder. Preferably,
this ~lending procedure takes place at a temperature
which does not antagonize the additives introduced.
Typically, this temperature is belsw the blending tem-
perature utilized to blend the high viscosity silicone
fluid and the primary thermopl~astic composition. The
blending temperature must be sufficiently high to main-
tain the viscosity of the polymers within the primary
thermoplastic composition and the secondary thermoplastic
compositic,l at a value that permi-ts them to flow and be
blended with other constituents. Minimum temperatures
for the polym~rs in the secondary composition are the
same as for those in the primary thermoplastic compo-
sition.
The weight ratio of the primary thermoplasticcomposition to the high viscosity silicone fluid prefer-
ably provides a value in the range of 100 to 1. Most
preferably the value of such a ratio falls in the range
of about 5 to about 2. It is often preferable to pro-
duce a blend which is predominantly thermoplastic poly-
mer in the initial blending s~ep to reduce the agitation
which is necessary to disperse the secondary thermoplas-
tic composition.
....... . . . . .... . . . . .. . . . . .... ..... ... ..... . . . .
.. ...
Rb-15,370
Preferably, the quantity of thermoplastic
polymers utilized in the secondary thermoplastic compo-
sition is sufficiently high to cool the blend within
the extruder to a temperature that permits the addi-
tives to be introduced without any harmful effect.
Therefore, the quantity of thermoplastic polymer util-
ized typically falls within the range of about 50~ to
about 1% of ~he total weight of ~he high viscosity 5il-
icone fluid and the primary thermoplastic composition
blended within the extruder. However, the quantity of
thermoplastic polymers within the secondary thermoplas-
tic composition must not be so high as to prevent ade-
quate mixing in the remaining portion of the extruder.
Thermoplastic polymers which are suitable for
use in this process include, for example, polycarbon-
ates, low density polyethylenes, high density poly-
ethylenes, polypropylene, polyphenylene ethers, poly
(alkyleneterephthalates), polystyrene, polyesters,
acrylonitrile, butydiene, styrene, polyethylene ethers-
polystyrene blends and copolymers, polybutylene, poly-
caprolactans, etc. Suitable acrylic polymers include
acetyl, ethylene, vinyl acetate, polymethyl-pentene,
~lex.ible polyvinyl-chloride, etc. It is not intended
that the above listing be all inclusive.
The h.igh viscosity silicone fluids are prin-
cipally comprised of high molecular weight silox~ne
polymers having viscosity values in the range of 90,000
centipoise and above at ambient temperature. The
siloxane polymers are tYpically compxised of chemically
co~.bined siloxy units selected from the group consist-
ing of:
R3Sioo 5
RR'SiO
R2 Sio
. R'~SiO
. . .... .....
z
RD-15,370
RiSiOl 5
~SiOl 5
R'R2SiOo 5 and SiO2 units,
wherein each R represents a saturated and unsaturated
monovalent hydrocarbon radical, R' represents a radical
such as R or a radical selected ~rom the group consist-
- ing of a hydrogen atom, hydroxy, alkoxyaryl, alyl, vinyl, aryl radical, etc. A preferred siloxane polymer is
polydimethylsiloxane having a viscosity of about 90,000
to 1,500,000 centipoise at 25 Centigrade.
Other constituents which may be found in the
high visc05ity silicone fluids include silicone resins
as defined by Frye in U.S. Patent 4,387,176. These are
typically characterized by the monomers within them.
For example, MQ resins are comprised oE M units of the
formula R3SiOo 5 and tetrafunctional ~ units of the
foxmula SiO2. An example of a suitable MQ silicone
resin is polytrimethylsilylsilicate, which can have a
ratio of M to Q units providing a value range of 0.3 to
~Ø Silicone resins containing other units such as the
trifunctional uni~ R~iOl 5, are also suitabl~. ~here a
silicone resin is utilized in the high viscosity sili-
cone ~luid a criteria for suitability is that each sill-
cone resin be soluble or dispersable within the mixture
of high molecular weight siloxane polymers that are
pre~ent so as to provide a homogeneous mixture. It is
preferable to premix the silicone resins and the
siloxane pol~mers prior to blending with the primary
thermoplastic composition within the extruder.
The preferred high viscosity silicone fluids
are those disclosed by Frye in U.S. Patent 4,387,176.
These silicone fluids typically contain a mixture of
high molecular weight siloxane polymers and one or more
silicone resins. An example of a high viscosity sili-
cone fluid disclosed by Frye is a mixture containing a
, . . . . . , . . . ... . .. ... . . . . . .. .. . .. . .. . , ~ . ., . , . .... . , , . . . .... ~ .
.. . .... .... .
RD-15,37~
silanol stopped polydimethylsiloxane pol~mer and poly-
trimethylsilylsilicate MQ resin.
The blend constituents which are added subse-
quent to the blending of the high viscosity silicone
fluid and the primary thermoplastic composition are typ-
ically sensitive to high temperatures. An example of
such a blend constituent is aluminu~ trihydrate. This
additive offers flame retardance to thermoplastic com-
positions by dehydrating when exposed to heat of the
combustion process. I~ exposed to high processing tem-
peratures, aluminum trihydrate can dehydrate in the
extruder and i~s flame retardance mechanism is lost.
Although this process is useful for introduc-
ing additives which are sensitive to the blending tem-
perature, additives which are insensitive to the blend-
ing temperature can a]so be introduced to the blend.
These additives may include, for example, re-
- inforcing fillers, cross-linking agentsl antioxidants,
lubricants (processing aids), flame retardants, etc.
Where the additives are insensitive to the processing
temperatures, they are often apportioned between the
molten thermoplAstic composition and the silicone
fluid when produced by a side fed operation~ Where the
blends are produced by a conventional continuous process,
the additives axe introduced into the feed hopper with
the pre-mixed high viscosity silicone fluid and primary
thermoplastic composition. Suitable additives which
can be found in the finished blend include those dis-
closed by Frye in U.S. Patent 4,387,176. These more
particularly include Group IIA metal organic compounds
or salts, such as magnesium stearate, calcium steaxate,
barium stearate, etc., which enhance the flame retaxd-
ance of the thermoplastic/silicone fluid blend. Other
flame retardants include the more conventional mater-
ials, such as antimony oxide and decabromodiphenyl
, . 11
.... .... . ..
~-15,370
oxide. Cross linking agents, such asdi~umyl peroxide
and primary reinforcing fillers, such as fume silica,
clay, talc,wollastonite,calcium carbonate, aluminum tri
hydrate, etc. can be fed into the feed hopper with the
solid prImary thermoplastic compositionO
When producing blends of polypropylene and
high viscosity silicone fluids having additives sensi-
tive to the blending temperature, the polypropylene is
preferably preblended with the desired additives which
are insensitive to the blending temperature where
these blends are produced by the side fed process disclosed
ln U.S. Pa-t. 4,446,090. These typically include, for e~-
ample, cross linking agents, reinforcing fillers, anti-
oxidants and processing aids.
The polypropylene and silicone fluid are typ-
ically blended at a temperature within the range o~
about 200 to 300 Centigrade, the preferred blending
temperature falling within the range of about 210 to
230 Centigrade~ Under these conditions, at most only
minor polypropylene degradation results and the dis-
persion of the high viscosity silicone fluid is suffi-
ciently high to provide excellent enyineering properties
(Impact resistance, tensile str~ngth, etc.).
An example of a desired constituent which is
sensitive to the pre~erred processing temperature range
of 210 to 230 Centigrade is aluminum rihydrate. This
additive dehydrates at these temperatures resulting in a
loss of its flame retardant capabilities. Although
other flame retardants are insensitive to such optimum
blending temperatures, aluminum trihydrate has been
found to offer improved flame retardance tG polypropy-
lene/silicone fluid blends when produced by this pro-
cess. Such blends typically exhibit short average burn
times of 5 seconds or less after exposure to a flame
test in accordance with Underwriters Laboratories, In-
corporated Bulletin UL-94.
12
.. . ... . .. .. .. . . ... . .
RD-15,370
The aluminum trihydrate is preferably intro-
duced into the extruder with additional polypropylene,
the quantity of polypropylene ranging from 1% to lOG~
by weight of the polypropylene/silicone fluid blend pro-
duced within the extruder. Most pre~erably, the quan-
tity of polypropylene is sufficiently high to reduce
the temperature o~ the thermoplastic/silicone fluid
blend within the extruder to a temperature within the
range of about 180 to 200 Centigrade. The tempera-
ture of the polypropylene not only cools the blend byconsuming sensible heat, it also cools the blend by
consuming the heat of fusion necessary to melt the
solid crystalline polymer.
The point of injection of the aluminum tri-
hydrate and polypropylene is about one~half to two-
thirds the extruder length from the feed hopper, the
point of injection being independent of the type of
process utilized to produce the thermoplastic/silicone
fluid blend. The preferred quantity of aluminum tri-
hydrate typically falls within the range of 0.1% to5% by weight of the finished blend.
The thermoplastic polymers utilized in the
secondary thermoplastic composition may be the same as,
but are not limited to the thermoplastic utilized in
the primaxy composition. When producing thermoplastic/
silicone fluid blends containing polyphenylene ether,
polystyrene and a high viscosity silicone fluid, poly-
phenylene ~ther may principally comprise the primary
thermoplastic composition and polypropylene ~ay prin-
cipally comprise the secondary composition,
The following examples are provided in orderthat thosP skilled in the art may be better able to
understand this invention. They are provided to illus-
trate this invention and are not intended to ~Lmit the
scope of this invention.
... . ...... " .. ..................... . .. . . .. .. ... . . .. .. ... . .... . ..... . ..... .. ... .... .. . . .. . .
t~ 2 RD-15,370
Exam~le I
A polypropylene/silicone fluid blend was produced
utilizing Werner and Pfleiderer Model ZSK-30 co-rotating twin
-i ~ screw extruder with intensive mixing screws 30 mm in diameter
and 29 diameters in length. A fu11y formulated blend was produced
in a single pass, said blend comprising 57.7 parts by weight
polypropylene, 8.5 par~s by weight high viscosity silicone ~luid
(comprising polydimethyl-siloxane and MQ resin in a ratio having
a value within the range of 1.9 to 1.0), 4.0 parts by weight
magnesium stearate~ 18.8 parts by weight aluminum trihydrate
~ATH) flame retardant and the remaining portion being decabromo-
diphenyl oxide.
The side ~eed process described in U.S. Patent Number
4,446,090, issued May 1, 1984 was utilized~ l~e pc~lypropylene
magnesium stearate, aluminum trihydrate and decabromodiphenyl-
oxide were introduced to the twin screw extruder through the
feed hopper. The polypropylene was melted within the extruder and
the silicone fluid was fed into the extruder at a point 1/3 of the
extruder length from the feedhopper. The high viscosity s;licone
fluid and the molten thermoplastic composition were blended at
various melt temperatures, including 188C, 198C, 2n40c and
215C ~t a scre~ speed of 300 RPM and throughput rate of 20 lbs/hr.
Four additional samples were run at a screw speed of 500 RPM and
throughput rate of 20 lbs/hr. at the temperatures indicated above.
The burn times for the eight blends are shown in Table I. The
burn times were taken in accordance with the test described in
Underwriters Laboratories, Inc. Bulletin UoL~~94
Table I
Melt temperature 188C 198C 204C 215C
Burn times (sec) 15 5 3.5 9
300 RPM screw speed
Burn times (sec) 18 7 5 6
500 RPM screw speed
. .
14
.. . . . ... ., .. .. .~ . .
~ B~ RD-l5,370
Example II
This example demonstrates embodiments of this invention
where the inte~rity of the blend constitutents that are sensitive
to the blending temperature of the high viscosity silicone fluid
and thermoplastic composi~ion is maintained. The blend samples
produced in this example had a similar composition to those
produced in ~xamp?e I, i.e. 57.7 parts by weight of polypropylene,
8.5 parts by weight high viscosity silicone fluid (polydimethyl-
siloxane and MQ resin), 4.0 parts by weight magnesium stearate,
18.8 parts by weight aluminum trihydrate (ATH) flame retardant
and the remaining portion being decabromodiphenyl oxide.
The same co-rota~ing twin screw extruder was utilized
and the side fed process disclosed in u.s. Patent 4,446,090
issued~ay 1`, 1984 was utilized to pro~uce the silicone
~ , .. ,.. , ~ ~
lS fluid/primary thermoplastic composition blend. The polypropylene
magnesium stearate and decabromodiphenyl oxide were introduced
into the feedhopper of the co rotating twin screw extruder, the
polypropylene being in an amount of 5Z weight percent of the
~inished blend. The polypropylene was melted within the extruder
and the high viscosity silicone fluid was introduced to the extruder
at a psint about l/3 the extruder length from the feedhopper.
Three polypropylene/silicone fluid blends were produced at melt
temperatures of about 200C, 205C; and 210C,respectively, and a
screw speed of 500 RPM within the extruder. The ATH was fed with
a portion of polypropylene, (5.7 weight percent of the finished
bl~nd) into the three blends downstream of the silicon iniection
part and blended in the extruder. The burn times for three
blends were taken in accordance with the test disclosed in Bulletin
U.L.-94 and are indicated in Table II along with the engineering
properties of the blends.
Table II
Melt temperatures 200C 205C 210O
Burn times (sec) 3.8 3.8 3.4
Tensile strength ~psi) 34lO 3412 3420
Gardner ;mpact ~in. lbs.) 148 165 184
,
.... ... ... .... .
6 ~ ~ RD~15,370
Example_III
This example demonstrates an alternative procedure where
po1ypropylene is not introduced with the ATH. The same three
blends were produced atd~fferent melt temperatures than Example
IIg howeuer, all the polypropylene was introduced into the feed-
hoppPr and the ATH was fed into the extruder downstream of the
silicone injection part without polypropylene.
The burn ti~es for the blends produced were taken in
accordance with the flamability test described in Bulletin U.L.-94.
1.0 The burn ~imes and engineering properties appear in Table III.
Table III
Melt te~peratures 190C 210C 220C
Burn times (sec) 15 7 9
Tensile strength (psi) 3450 344~ 3465
lS Gardner Impact (in lbs.) 1 zn 50 160
The data in Table III illustrates the erratic burn times
obtained by this process indicating dehydration of ATH results.
Although the above examples have shown various
modifications of the present invention, further modi~i-
cations are possible in light of the above teaching byone skilled in the art without departin~ ~rom the
spiri~ and scope o~ ~he inventi~n.
. 16
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