Note: Descriptions are shown in the official language in which they were submitted.
~i3~'7
The present invention relates to a thermal insulant
material for high-temperature applications, and in particular to
an (asbestos-free) insulant for rocket motors.
secause of the associate health hazard, asbestos is
subject to restrictive regulations which present a serious
handicap to the marketing of asbestos-containing insulants.
One asbestos-containing insulant for rocket motors is
described in applicant's United States Patent No. 3,872,205 ~hich
issued March 18, 1975. The patented material comprises an
elastomeric polymer binder, particularly a carboxyl-terminated
polybutadiene, having dispersed therein an asbestos reinforcing
filler material. At least 50% of the asbestos is in the form of
fibres which provide for reinforcement. The remainder is in the
form of floats (chunks) which act as a filler.
In high temperature applications, such as a flexible
sheet like liner for a rocket motor casing or a molded head-end
insulator for a rocket motor casing, various requirements must be
satisfied. Some of these requirements are:
- suitability for sheet-milling;
- compatibility with existing manufacturing processes
and methods used inside rocket motors;
- self-adhesive to metal motor casing wall;
- an acceptable shelf life at -15C;
- good mechanical properties; and
- suitability for compression molding at room
temperature.
The asbestos-containing insulant described in Gur
aforementioned United States Patent meets these requirements
. .. , ~.,,,, --1--
32~
because the asbestos serves as both a filler and provides
reinforcement.
It i5 thus an object of the invention to provide an
asbestos-free thermal insulant material which exhibits the
requisite properties for use in rocket motors.
According to the invention, a novel thermal insulant for
use in a rocket motor, said rocket motor including a metal casing
and a solid propellant, said insulant comprising a carboxyl-
terminated polybutadiene based binder, a thermally stable,
reinforcing organic fibre material comprising a mixture of an
aromatic polyamide fibre material and a heat-stabilized
polyacrylonitrile fiber material, and a powdered siliceous filler
material, other than asbestos wherein both the reinforcing fibre
material and the filler material are compatible with the binder
and wherein the thermal insulant is self-adhesive to said metal
casing. We know Erom experience that even though the fillers and
fibres are theoretically inert, one may encounter incompatibility
problems with the resin. This would mean being faced with either
a longer curing period or worse yet no curing at all. Hence,
compatibility of fibres and fillers is in the sense of not
appreciably affecting the curing time of the resin.
More specifically, our invention involves the use of a
novel reinforcing fibre/filler composition. The filler is a
siliceous material such as Kaolin, mica and talc. Kaolin
(snobrite grade) is preferred. The reinforcing fibre is
preferably Aramid fibre. Kevlar~ 29, dry pulp 979, an aromatic
polyamide fibre, manufactured by Dupont is most preferred.
Optionally, a second thermally stable fibre material may also be
-2-
included, such as an inorganic Eibre, e.g. Refrasil (fibre grade
F100-A-25, made by Hitco) which is a silica fibre, Saffil~ and
Fiber-fra~ , which are both ceramic fibres made from alumina and
silica; and organic fibres such as Pyron (a trade name for heat
stabilized polyacrylonitrile fibre, manufac-tured by Stackpole
Fibers Co.
Our patented asbestos-containing insulan-t, described
above contains about 70%/W of solids (asbestos), 50-80% of which
is in the form of -fibre~s and 20-50% is in the form of floats. The
total solids loading of the new formulation varies from 55-70%/W
comprised of 25-65% of powdered siliceous material and 35-75% of
fibres. Kevlar~ is present between 14 and 40%/W of the solids and
Refrasil~ may vary from 25 to 55~/W of the solids.
Table I, below, describes the compositions of the novel
insulants according to the invention by formulations numbered
(890, 891, 892, 1005, 100~ and 1021). RF/B is the designation for
our paten-ted asbestos-containing insulant described above and has
been included for comparison purposes.
TABLE_I
Insulant compositons
FormulationsRF/B 890 891 892 1005 1008 1021
Ingredients % Wt
Binder formulations
HC-43~4/PBNA94.09 95 95 95.793.09 93.14 93.09
Erla ~ 05104.93 5 5 4~34.584.594.58
Iron Octoate0.98 - - .33 .27 2.33
100 100 '100 100 100 100 100
Epoxy/HC Ratio1/1 1/1 1/1 0.85/1 1/1 1/1 1/1
Binder as above 30 40 42 40 43 44 31
Asbestos 3Z12/7TF1 70
Kevlar 29 (Dry Pulp
Type 979) - 10 20 2220 10 10
Kaolin - 34.5 15 37.520 31 22
Refrasil tF100-A-25) - - 22.5 - 17 - 37
Pyron (Half-inch
cut fibres) - 15 - - - 15
Mg(OH)2
(Brucite) - 0,5 0,5 0.5
100 100 100 100100100 '100
';~.
3~2~i32~t7
The carboxyl-terminated polybutadiene-based binder of
the new formulations contains the same HC-434 (manufactured by
Thiokol) polybutadiene polymer (including 1% PBNA antioxidant) and
the same ERLA-0510 epoxide curing agent (manufactured by UniGn
Carbide) that were used in the RF/B . There are some variations
however in the cure catalyst; the iron octoate (IO) is preferred
in the new formulations and the concentrations are different of
what it was in the RF/B. Formulations 890, 891 and 892 use
Mg(O~I)2 as cure catalyst at the rate of 0.5% by weight of
composition. It should be noted that at the time of the patent,
the curing catal~st for RF/B was iron octasol. Iron octoate is
now preferred and used in the RF/B as well as in the new (1005,
1008 and 1021) formulations.
Brucite (Mg(OH)2) was used in formulations 890, 891
and 892 as the cure catalyst; a rigorous particle size control is
re~uired when usin~ this material to obtain a reliable catalytic
effect. The formulations are based on a lot having an average
particle size of about 3.0 microns.
Ingredients Preparation
~ . _
In the case of formulations 890-891-892, the solid
ingredients were used as received. Formulations 1005-1008 and
1021, however have been optimized using dried solids to obtain
reliable curing rates~ The solids were (dried) heated at 100C
for a duration of 15 hours.
Insulant Preparation
~ . .
The following procedure was developed for the new
insulant formulations containing Kelvar , using a 800 q
sigma-blade mi~er; the same cycle was used with a 20 kg sigma
.~63~
mixer with good results. It should be noted that the duration of
step 2 as well as steps 3 to 7 have contributed to improved mxing
and that the cleanliness of the mixer at the end of the mixing
cycle is related to how closely one fo]lows that cycle. Steps 3
to 6 included are to be accomplished over a period of
approximately 30 minutes.
1. Load the polymer (~IC-434) and the curing agent
~ERLA) and mix for 5 min~
2. Add the catalyst (IO) and mix for one minuteS
3. Add, in small increments, one third of the Kelvar
fibers,
4. Add in small increments, half of the Kaolin and half
of the Refrasil (if included) by switching from
one to the other throughout the whole step.
5. Repeat step 3, step 4 and step 3.
6. Mix until all solids are well dispersed.
7. Transfer the dough to an oven at 70C and cure to a
Shore "~" hardness of 50 to 60 or until processable
on a differential mill.
8. The partially cured dough is then processed through
a differential speed rolling mill to get a homo-
geneous blanket. After about 15 passes, the blanket
no longer sticks to the rolls.
9. The last step consists of three passes on the final
sheeting mill as follows;
1 - long fold
2 - book fold
3 - long fold
-6-
This yields a smooth and uniform sheet of the
desired thickness.
10. To maintain the shelf-life of the new insulant
material it is recommended to store it at -15C.
Compositions 890, 891 and 892 do not use IO but
Mg(OH)2 which is first dispersed into the Kaolin and incor-
porated to the mix with the Kaolin. When Pyron~ is used (890,
1008) instead of Refrasil~, it is added slowly after Step 5.
Formulation 1021 is slightly different; having a reduced
binder content (31% by weight compared to at least 40 for the
others), it is preferably milled (Step 8) when the Shore "A" hard-
ness is only about 30.
It should be noted that Steps 7 to 10 of the above cycle,
apply also to RF/B; that portion of the insulating preparation
has not been modified.
MOLDI~IG INSULATORS AND ROCRE~' MOTOR INSULATION
Applicant's U.S. Patent No. 4,108,940 which issued
22 August 1978, covers the cold (room-temperature) molding of
insulant component parts for rocket motors such as head-end
insulators, from shreaded RF/B. The same technique can be used
for molding motor parts with the new asbestos-free formulations.
The installation into a rocket motor casing is achieved
by low pressure bag molding and is described in our U.S. Patent
No. 4,148,675 which issued 10 April 1979. Again, the technique is
the same whether we use RF/B or any one of the new formulations.
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M~CHANI CAL PROPERTI ES
Typical mechanical properties of formulation 1005 for
fresh product as well as for the product after 28 and 56 days of
storage are presented in Table II. Mechanical properties of
formulations 890, ~91, ~92, 1008 and 1021 are listed in Tables
III - VII, respectivelyO Generally speaking, the mechanical
properties of formulations 890, 891 and ~92 are better than those
of 1005. These three formulations exhibit higher machine and
cross machine direction stress and the raw material's stability
under storage conditions is better. Moreover, there is no
evidence of a change in mechanical properties. However, curing
inconsistencies occurred when we changed brucite lots later on.
Formulation 1021 was rejected because of a reduced adhesive stress
to metal~ Mechanical properties of formulation 1008 are slightly
inferior to those of 1005. Hence, formulation 1005 is preferred~
Tensile Tests
Tensile tests were performed on (2.5 cm x 10 cm x
0.6 mm) samples with an Instron tester, at a crosshead speed of
1.27 cm/min. For each formulation, specimens were tested as
follows: fresh (as rolled), after 1 hour of curing at 60C and
after 7 days of curing at 60C. The same tests were repeated with
samples stored for 2~ and 56 days at -15~C as shown in Table II
Adhesion Tests
Bond tests were carried out using the procedure TP-P-14.
According to this procedure, the material is first punched into
wafers and the test specimens are then prepared by pressing them
between two steel cylinders (9.0 sq. cm) at a pressure of 900 kPa
for 1 hour at 60C; then the specimens are cured for 7 days at
60C before testing.
,.. ~.
~JI., _ 9 _
ROCKET MOTOR FIRINGS
Six CRV-7 motors loaded with a solid propellant (C-15)
and insulated with the 1005 formulation were fired and they
performed normally. Two were fired at room temperature while the
others were s~bmitted to thermal shock cycling (5 cycles, -54C
and +65C). Of these, two were fired at -54~C and two at +65C.
X-ray examination of the insulant-casing interface after thermal
shock treatment did not show any difference from that done before
treatment, The latter test demonstrates the suitability of the
adhesive stress of the formulation.
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