CA1263227A - Asbestos-free insulant for rocket motors - Google Patents

Abstract

ABSTRACT

The invention disclosed is a thermal insulant for high-temperature applications such as in rocket motors. The novel insulant is asbestos-free and comprises a carboxyl-terminated polybutadiene-based binder, a thermally stable, reinforcing fibre material, preferably an aromatic polyamide fibre material, and a powdered siliceous filler material other than asbestos, wherein the fibre and filler materials are both compatible with the binder.

Description

~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 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.

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~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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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A 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 fibre material, and a powdered siliceous filler other than asbestos, wherein the reinforcing fibre material and filler material are compatible with the binder and wherein the thermal insulant is self-adhesive to said metal casing.

2. A thermal insulant according to claim 1, wherein the total solids loading comprises 55-70%/W of the composition.

3. A thermal insulant according to claim 2, wherein the solids loading comprises 25-65% of the filler material and 35-75%
of the reinforcing fibre material.

4. A thermal insulant according to claim 1, or 2, wherein the siliceous filler is kaolin.

5. A flexible sheet thermal insulant for use in a rocket motor, said rocket motor including a metal casing and a solid propellant, wherein said insulant is disposed between said casing and said propellant, said insulant being self-adhesive to said casing, said insulant comprising 30-45%/W of a carboxyl-terminated polybutadiene-based binder having dispersed therein 70-55%/W of a combined reinforcing fibre/filler material, wherein said fibre is a thermally stable, reinforcing organic fibre material comprising a mixture of an aromatic polyamide fibre material and a heat-stabilized polyacrylonitrile fiber material, and said filler is a powdered siliceous filler other than asbestos.

6. A thermal insulant according to claim 5, wherein the filler is kaolin.