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WALTER C. HOURT Atlantic Research Corp., Alexandria, Vai
Plastics as Heat Insulators in Rocket Motors Solution to a mathematical model describing thermal degradation of plastic materials showed that observable performance variables are functions of the thermal properties of the insulator
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N ITS SIMPLEST form, a rocket motor is a vessel where reactions occur a t temperatures of several thousands of degrees and pressures sometimes exceeding 100 atm. Few construction materials can withstand these extreme conditions, and as a result, a heat insulator must be used. At first glance, plastics seem the least useful, but if their destruction is controlled, they are particularly suited for solid-propellant rocket motorsthey have the unique property of uniform rather than catastrophic destruction. Recently, plastics have been used for re-entry nose cones and structural parts ; in fact, an almost completely plastic rocket is an actuality. However, the higher temperatures and pressures which are necessary for longer range rockets mean that efficiency of plastics as heat shields must be improved. This can be done by empirical and theoretical studies which lead to a more thorough understanding of how these materials behave in the rocket environment. Conditions in solid propellant rocket chambers can vary widely, but in the future, plastic insulators may be exposed to temperatures ranging from 3000' to 9000'F. Improved design configurations and longer flight ranges require longer burning times and, therefore, exposures of several minutes to elevated temperatures. Motor pressures may range from 200 to 3000 p.s.i. with a broad range of gas densities. In a single motor, bulk velocity of the
The key to the problem of using plastics as heat shields in longer range rockets lies in controlling properties of the char which they must form under the high temperatures and pressures inevitably encountered. This char, held to the substrate by a reinforcement, must be strong enough to resist erosion caused by gas-flow stresses, and yet be porous enough to allow the products of substrate decomposition to pass through it. Also, for minimum thermal conductivity, it should be amorphous rather than graphitic, and it should form a heat-transfer medium for cracking reactions.
gas stream can vary from subsonic to as high as Mach 6. Then added to these problems is chemical reactivity of the extremely hot propellant products with components of the plastic insulators. The effect which these parameters have upon plastic heat insulators is exceedingly difficult to discern in rocket motor tests. There is a complex interaction which occurs between the competing rate processes for surface ablation and resin pyrolysis. Depolymerization, melting, vaporization, pyrolysis, and pyrolytic cracking take place, and the rates of these processes increase
as temperature is increased. The pressure in the rocket chamber is generally sufficient to fracture a rigid-type plastic insulation. Many materials blow or expand because of entrapped gaseous decomposition products and if the insulation is a laminated plastic this can result in destructive delamination. At high pressure, expansion is less apt to occur, and pyrolysis yields a hard char layer. Heat conduction within the insulation is affected by pressure through transpiration of decomposition gases and heat transfer coefficients. The exhausting gas stream impresses a shear stress on the plastic insulation which is manifest by surface erosion of the pyrolyzing insulation. Although the magnitude of surface erosion depends on specific composition of the plastic insulator, nevertheless, erosion increases with increased rate of gas flow. Gas turbulence a t high velocity leads to hot spots and thin boundary layers with high heat-transfer coefficients and greater heat transmission through the insulation. Chemical interaction of the propellant combustion products with the residue of pyrolyzed insulation-e.g., oxidation of the carbonaceous char by C O Z and HzOis manifest by drastic changes in the rate of surface ablation. The reason some plastics work under such degenerative exposure conditions and are preferred to other materials lies in a unique combination of properties which reinforced plastics possess. VOL. 52, NO. 9
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SEPTEMBER 1960
761
Thermal and mechanical shock resistance, low thermal conductivity, high endothermic heats of transformation, self cooling, char formation, and slow uniform surface ablation contribute most significantly to successful insulation performance. Rocket motors on ignition attain operating pressure and temperature within a few milliseconds, conditions generally fatal to shocksensitive materials such as ceramics. The thermal and mechanical shock resistance of plastic insulation materially reduces the chance of breaching the insulation wall. Low thermal conductivity causes a steep thermal gradient within the insulator and a narrow charring zone which eliminates the possibility of widespread destruction and the necessity for great thicknesses of material. The plastic matrix, the reinforcement, and filler components absorb large quantities of heat energy during pyrolysis, melting, vaporization, and pyrolytic cracking, which act as blockades to heat transmission and lessen the rate of interior deterioration. The formation of a hard carbonaceous surface during pyrolysis is vital because it resists thermal and mechanical ablation, and chemical attack. Gaseous products of decomposition transpire through this char layer and scavenge additional heat; first, by endothermic decomposition; second, by flow of the cool gas directly opposed to the flow of heat; and third, by injection into the boundary layer. Extensive evaluation testing has shown that for most rocket motor applications the char-forming reinforced plastic insulator is more efficient on a protection-us.-weight basis than a completely ablating insulator. The mechanistic behavior of plastic composites during severe heat is a complex interaction of surface ablation, pyrolysis, and heat transmission. Studies on specific materials in a plasma jet. for example, indicate that there is some temperature, in the neighborhood of 6000" F., at which the rate of surface ablation and the rate of pyrolysis are equivalent. Below this temperature, pyrolysis is the controlling reaction, and above 6000" F. surface ablation has first consideration. In all probability such a relationship exists for all plastic insulators and each of the previously mentioned environmental parameters has its effect upon the conditions a t which there is rate equivalence. If we expect to find the optimum insulator we must ask ourselves what components of the reinforced plastic insulation and what properties of these components are most significant in successful performance. Efforts to answer these questions are still in process. Observations of the behavior of reinforced plastics in rocket motor tests and in simulated environment tests in
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the laboratory have produced a fairly clear picture of the macroscopic factors involved. There are four major reaction areas: the propellant gas-solid heating zone, the pyrolyzed residue, the pyrolysis zone, and the unaltered plastic insulation. A heat-conduction analysis of these zones was made for a system in which
The resultant equations and boundary conditions are
T = T,atx=O,t>O T=Oatx>O,t=O
1.
The surface of the insulation was instantaneously heated to the flame temperature of the propellant combustion products. 2. The total thickness of the insulation did not change. 3. The plastic matrix was pyrolyzed a t a specific temperature and with a specific absorption of energy. 4. A stable layer of charred material was formed on the flame side of the pyrolysis zone. 5. The unaltered insulation had low thermal conductivity. 6 . The gaseous products of pyrolysis had no cooling effect. This condition can be omitted if effective thermal constants for the pyrolyzed laver are used. A few more conditions were assumed for mathematical simplicity. 7. heat flow. which is 8. Lnidirectional Semi-infinite thickness _
I
T+Oatx>aasx+ x
lim x+
Dimension x
0
(-KI
+-a +
x -+ a-
g)
a+
= lim
(
-
~
2
2)+-
%+a-
da U(.
(7)
where Tis temperature at some location in the insulation; t , time of exposure; a , thermal diffusivity ; x , position coordinate ( x = 0 corresponds to heated surfaces); T,, temperature at heated surface; To, temperature of decomposition of plastic insulator; X, thermal conductivity; L , latent heat of pyrolysis of plastic insulator; p, density; and a, position coordinate of Wrolysis *One* Subscripts such as 1, 2, and 3 indicate the reaction zone in which the variable has meaning. Equation 6 specifies that a t the pyrolysis zone decomposition takes place at temperature Tu, and that as this zone is approached from
/'-
-'
(5) (6)
L P 2 si;
quite valid for time iIlterval of interest. 9. Thermal conxants are invariant lvith respect to temperature.
7
(4) m
lim ( T ) = lim ( T ) = T.
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Carbon plus reinforcement residues
(3)
Resin pyrolysis
Unaltered 7 insulation
,-Wall
; 7 d x ) x=a
Temp era t u re (time fixed)
0 Pyrolysis in reinforced insulation materials. Raising the temperature a t which pyrolysis occurs affects only slightly the r a t e a t which the insulator deteriorates
INDUSTRIAL AND ENGINEERING CHEMISTRY
P L A S T I C S A N D ELASTOMERS IN ROCKETS either side the temperature must approach T a . Equation 7 specifies a similar equality for the heat flux at the pyrolysis zone. The mathematical complexities of this problem are not of real concern and the solution obtained is: 4 =
Rdi
(8)
where R, the char-rate constant, in approximate form is
Subsequently, however, a more exact and simplified approximate form for R was calculated :
The equation a = R@ states that the depth of penetration of the pyrolysis zone into the insulation is equal to a constant times the square root of the exposure time in the high temperature environment. The char-rate constant reflects the significance of the properties of a specific plastic insulator. The dependency of R, the char-rate constant, upon these variables was assessed by assigning arbitrary values to each variable and computing R. For example :
K,
= 0.006
0.05 0.05
0.01
0.01 0.01 0.01 0.01 0.05
cal./sq. om. sec./O C./cm.
012
= 0.002 sa. cm./sec.
p2
=
Ki (Cal./Sq. Cm./ (Y1 Sec./OC./Cm.) (Sq.Cm./Sec.) 0.01 0.01 0.01 0.01
transformation temperature considerably above that of the plastic matrix. Consequently, high temperature exposure eventually produces a multilayer structure with several decomposition zones. This model system also was studied analytically. The analysis was more complicated and solutions for decomposition rates were obtained as coupled equations-Le., the rates of transformation of the reinforcement and pyrolysis of the plastic matrix were interdependent. Nevertheless, the results bear out the preceding conclusions. The theoretical analysis and rocket motor studies highlight the essential properties for plastic heat insulators. High temperature degradation must form a carbonaceous or other stable residue. This char must be hard and strong to resist erosion caused by gas flow stresses, it must be porous to allow transpiration of the products of substrate decomposition, and it should form a heat-transfer medium for “cracking” reactions. The carbonaceous char should be amorphous rather than graphitic to achieve minimum thermal conductivity. The reinforcement should hold the char layer to the substrate and undergo endothermic transformations. Endothermic reactions and transpiration of products should be maximized to obtain optimum absorption of heat energy. Materials of low thermal conductivity must be used to retain high insulation quality in the substrate insulation layers.
1.4g./eo.
Ta
Tr
I 4
(” C.)
(” C.)
(CalJG.1
400 400
800
3000 2000 3000
400 400 400
3000 3000 3000
3000 3000 3000 500
These results show that degeneration within a plastic insulator is particularly sensitive to the quantity of heat absorbed during pyrolysis and to the thermal constants of the char residue. Lowering the exposed surface temperature lowers the rate of charring, but raising the temperature at which pyrolysis occurs has onIy slight influence on the rate at which the insulator deteriorates. The thermal properties of the unaltered insulation appear to have minor influence, but it must be borne in mind that their value must be low to maintain a steep thermal gradient at the pyrolysis zone in order to satisfy the initial postulates. In reinforced plastic insulators the reinforcement or filler frequently has a
3000 3000
Id (Cm./Sec.’/r) 0.108 0.087 0.101 0.220 0.205 0.250
High-yield char-forming plastics are generally highly cross-linked thermoset resins of the phenolic, epoxy-anhydride, polytrivinylbenzene, and polyacrylonitrile type. Generally melamine, ureaformaldehyde, and thermoplastic resins are low-yield char formers. Microporous char structures may be achieved by incorporating decomposable fillers which have high heats of decomposition or vaporization and react at the right temperature stage during the carbonizing process. A fine balance is needed to equilibrate the competitive processes of stable residue formation and extensive gas production for optimum performance. One approach to achieving this balance
led to reinvestigation of ceramic and refractory materials. A basic deficiency of ceramics and refractories is their thermal and mechanical shock sensitivity. If this sensitivity could be obviated, then their moderate thermal conductivity, which is lower than amorphous carbon in most cases, their high melting point and fusion energy, and low emissivity could be advantageous in heat insulation. To clarify the terminology, materials of interest might include the metal oxides, nitrides, carbides, zirconates, and titanates. Incorporating such materials in a plastic bonding agent does in fact eliminate their thermal and impact sensitivity and forms a carbon-bonded structure at high temperature. The ceramic or refractory may be the continuous phase, or a discontinuous phase in a continuousplastic matrix which is a high-yield charforming resin. Each combination produces some interesting insulation composites which embodied one or more of the essential properties mentioned earlier. The discontinuous ceramicphase composite yields some measure of improved performance over that of the basic unfilled insulation. The continuous-phase composite has yielded some interesting but not completely consistent results. Evaluation of such composites in an oxyacetylene Aame has shown that some combinations have significantly better erosion and thermal resistance than graphite and certainly much lower thermal conductivity. However, it has also been found that these materials are not as resistant as graphite in a rocket chamber test. The future progressive development of efficient plastic heat insulators requires detailed understanding of the basic principles of performance. Mechanism studies of high-temperature insulation are essential if we are to cope successfully with the coming age of ultra-high temperatures. There is a present need for results from fundamental studies in high-temperature chemistry, studies of the rates of depolymerization and pyrolysis, and of the relation of polymer structure to such rates. We need to know more about the high-temperature thermochemical properties of our materials; their phase change reactions, enthalpy, and heats of reaction. The mechanistic picture is emerging, and the necessity for these basic studies has become imperative. RECEIVED for review November 6, 1959 ACCEPTEDJune 6, 1960 Division of Paint, Plastics, and Printing Ink Chemistry, Symposium on Plastics and Elastomers for Use in Rockets, 136th Meeting, ACS, Atlantic City, N. J., September 1959. VOL. 52, NO. 9
SEPTEMBER 1960
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