Evaluation methods for ablative plastics. A review of the techniques used

Donald L. Schmidt and Herbert S. Schwartz Nonmetallic Materials Division, Air Force Materials Laboratory, Research 6. Technology Dizjision, Wright-Patterson AFB, Ohio

Eva1 u at ion Methods

for Ablative Plastics

A review of the techniques used

MICRO-SECOND X-RAY SOURCE

FLOW REGULATOR

FLOW METERS PRESSURE GAUGES

HELIUM

Figure 1. Schematic of a refined oxy-acetylene torch facility.

I n the selection of an ablative material for a new system, preliminary studies or design studies must normally make use of data that have been obtained previously. In the simplest procedure, the total heat input is computed and is divided by the experimentally obtained “heat of ablation” value of a number of representative plastics to determine how much material is needed. This procedure is meaning- ful only if the ”heat of ablation” values used were obtained under experimental conditions identical to those anticipated for the system under consideration. In fact, one may find “heat of ablation” values in the literature for a given material, which differ by an order of magnitude. The reasons for such a wide range of values are that “heats of ablation” or similar performance indices are not intrinsic materials properties, but depend on subtle thermal, chemical, and physical interactions of the material with the environment. The variation of ablation performance with environment may be considered analogous to the variation of rupture strength of a material with rate of loading, duration of applied load, or environmental temperatures.

The problem of evaluating performance of ablative plastics may be attacked by two basic approaches. One consists of obtaining overall performance data on a number of representative materials under environmental conditions repre- senting those anticipated in service. This approach is slanted toward specific solutions and does not elucidate the basic materials characteristics leading to generalized performance prediction equations. The other approach consists of funda- mental and applied research to establish qualitatively and quantitatively the individual aspects of materials-environment interaction. hlathematical expressions are written to define total performance, such as material erosion and temperature distribution in terms of the integrated individual

phenomena involved. Depending on the specific objectives, both approaches have merits. The first is shorter range and evokes more confidence because the material performance may be directly observed. The second furnishes greater in- sight into the ablation mechanisms and provides a firmer foundation for selection of candidate materials for detailed consideration and for future materials synthesis.

Many of the experimental evaluations and studies on ablative plastics are not made under true environmental simulation conditions, but they are still very useful. They help define the physjcal and chemical mode of ablation, investigate certain critical environmental parameters, and furnish materials property information for analytical equations predicting performance.

ENVIRONMENT INFLUENCES ABLATIVE PERFORMANCE

Environmental variables and their magnitudes strongly influence the performance of an ablating plastic. Extensive investigations on this material-environment interrelationship reveal that the importalit parameters are thermal, mechanical, and chemical in nature (1, 2 ) . These critical environment variables have been further defined, and they are shown in Table 1.

Thermal Parameters-Ablative plastics intended primarily for use in convective heating environments should be evaluated using the same mode of incident heating. Similarly, plastics which may be subjected to intense radiant energy should be characterized in a radiant heating environment. The type of behavior exhibited by a material in each type heating condition is likely to differ significantly. For example, plastics that gasify in response to heating effectively

SPE TRANSACTIONS, OCTOBER, 1963 238

block a large fraction of incident convective heating by mass transfer (of gases) in the opposite direction. These pyrolytic vapors will have little effect on radiant energy input, if present, since thc gases are essentially radiation transparent. Radiant energy may also penetrate deeply into a transparent, ablating material and result in high backwall tcmperatures. The behavior of ablative plastics is altered greatly by the incident heat flux. At low heating rates, the performance is dictated primarily by the material's thermophysical properties. The plastic will behave like a heat sink, unless the thermal degradative processes are forced by use of a low temperature ablator. At higher rates of heating, the material performance is collectively determined by both the ablative characteristics and the thermophysical properties. At very high heating rates, the ablative characteristics exert a predominant influence on material performance. The variation in heat flux with time is another important environmental variable. If the incident flux is nonvariant, a homogeneous or composite material will likely exhibit optimum performance. Materials intended for environments having a variable heat flux should probably be graded in the thickness of the specimen, thus matching the instantaneous heat flux with the optimum material composition. The total heat load and heating time collectively dictate the amount of material necessary for thermal ablation and insulation. The temperature or enthalpy of the environment is also important. The thermal efficiency of an ablating plastic increases (up to several fold) with higher gas enthalpies. It is thus inappropriate to characterize plastics destined for high enthalpy, re-entry environments in a low enthalpy, combustion gas medium.

Mechanical Forces-The imposed mechanical forces act- ing on an ablating plastic may adversely alter its performance in an unpredictable manner. With increasingly higher stagnation pressures, the thermally weakened surface material tends to be crushed and swept away before it is utilized. High velocity gases sweeping across a material surface ( a rocket nozzle), produce very high interface shear stresses. These induced stresses may be sufficiently large to mechanically scrub particle matter from the surface, or increase liquid sloughing and run-off. Phenolic-nylon has been observed to lose its surface char entirely in high mass-flow rocket exhaust, whereas in subsonic arc heated air, it forms and retains a thick, protective char layer. Ablating surfaces may also be mechanically abraded by energetic particles contained in the gas stream. Combustion gases from solid propellant motors frequently contain en- trained liquid and solid aluminum (or alumina). Impact of these particles on the ablating nozzle wall results in high

Table 1. Environment Influences Ablative Performance

Thermal Parameters Mechanical Parameters Chemical Parameters

Temperature Deceleration Gas Enthalpy Acceleration Mode of Heat Transfer Pressure Reactivity Total Heat Load Shear Oxidation Shape of Heat Pulse Abrasion Reduction Peak Heating Rate Vibration Heating Time

SPE TRANSACTIONS, OCTOBER, 1963

attrition by fluxing of the molten layer or particle shear. Mechanical and accoustical vibration may produce de- tachment of ablating surface material. This effect un- doubtedly occurs in the plastic expansion cones of solid propellant rockets, but, to date, it has not been studied experimentally in the laboratory. Inertial forces of acceleration and deceleration may alter the flow of surface melt on an ablating plastic, thus producing first order changes in surface geometry. These effects have been noted on ablating re-entry nose cones, but they have not been in- vestigated experimentally due to the complexity involved.

Chemical Parameters-Test environments often contain reactive chemical species, which may combine exothermally with the ablating surface and accelerate material vaporization. Thermochemical attack of ablating surfaces by oxidizing and reducing gaseous species has been noted in many different test environments. Charring plastics are particu- larly susceptible to the oxygen content of a test environment. For example, arc heated oxygen produces ablation rates of phenolic-nylon up to twenty-five times higher than those involving an inert gas. High temperature water vapor present in the combustion products of solid propellants may also cause appreciable oxidation of ablating carbonaceous surfaces ( 3 ) .

ENVIRONMENTAL EXPOSURE TESTS The characterization of ablative plastics involves the

careful selection of an appropriate test facility. In carry- ing out this function, it is necessary to give consideration to many factors, such as: the availability and cost of the material to be characterized, the extent and accuracy of the information desired, the environmental conditions desired and ability to generate them simultaneously, degree of control over the environmental parameters and ability to vary them individually, uniformity and reproducibility of the test medium, and ability to calibrate accurately the test medium.

Laboratory devices used in characterizing ablative plastics may be arbitrarily classified into four basic types: scrcen- ing, specialized environmental, simulation, and nonsimulation facilities. In the following, each laboratory device and associated test procedure shall be described in detail.

Screening Facilities The primary purpose of screening facilities is to identify

materials compositions and constructions which exhibit de- sirable ablative characteristics and thus warrant further evaluation, and reject other undesirable materials early in the evaluation scheme. It is necessary, first, to characterize all candidate materials in these facilities. Combustion gas torches and arc-plasma units have been widely used for screening, since they are readily available, relatively inex- pensive, easy to operate, and give reproducible results. Materials intended for high enthalpy or high heat flux environments are characterized with an arc-plasma unit. Mate- rials intended for use in combustion gas environments are exposed in a similar type of environment. Both genera1 materials behavior and ablative characteristics are obtained in this way (4).

Combustion Gas Torch-Small gas torches utilizing a wide variety of fuel-oxidant mixtures are used to generate high temperature test media. The oxy-acetylene torch (5 to 9) is perhaps the most widely employed materials screening device, although oxy-kerosene ( lo) , oxy-city gas (11) ,

239

oxy-hydrogen ( 12), oxy-propane, and oxy-cyanogen torches have also been employed.

The general performance characteristics of a subsonic oxy-acetylene torch are given in Table 2. The facility is basically composed of a hand operated torch and a gas metering system, or a fixed torch arrangement (13). Refine- ments of this screening device have been made, one of which is schematically illustrated (14) in Figure 1 . Test materials are prepared in standardized sizes and shapes, in the forms of flat-face cylindrical rods, wedges, or rectangu- lar pieces. A thermocouple is affixed to the back-face of the specimen, or at a predetermined position in the material substrate. The specimen is mounted in an insulated holder to insure one-sided heating and prevent any possible side- heating effects. After ignition and stabilization of the combustion flame, the specimen is rapidly swung into exposure position. Instrumentation available for measuring the response of the material is actuated simultaneously with specimen exposure. The desired surface heating rate and stagnation pressure are obtained by selecting a proper distance between the torch tip and the specimen front. If the specimen is positioned too close to the torch tip, the flame may be extinguished or unstable combustion may result. During exposure to the hot gases, the surface of the specimen re- cedes (ablates). It may be necessary to continuously ad- vance the specimen front to its initial exposure position to maintain nonvariant environmental test conditions. The rate at which the specimen front is advanced is identical to the material linear rate of ablation. Temperature data

from the thermocoupIe(s) are recorded throughout the exposure period, so that the material insulation index may be determined. Specimens are exposed from about 30 to 120 sec, unless mechanical failure of the test material occurs sooner. If this occurs the specimen is quickly removed from the flame front and rapidly cooled. Post-exposure examination of the specimen surface and cross-section yields general information on the material's behavior at high temperatures, char depth, and similar data.

If the test materials are destined for use in combustion gas environments of high pressure and shear forces, they should be screened in supersonic combustion flames.

Electric Arc-Jets-The gas-stabilized electric-arc heater is a laboratory tool frequently used to determine both transient and steady-state ablative characteristics of plastics. It may be operated with virtually any gas, thus providing considerable versatility in gas chemistry. The arc-jet has been widely used to simulate the thermochemical effects of atmospheric re-entry (15 to 24) and the high-temperature combustion products of rocket motors (25).

A photograph of a typical 250 kilowatt (2.5 x 1OI2 erg/ sec) arc-jet unit exhausting into the open atmosphere is shown in Figure 2. It consists of two electrodes and associated water-cooled chamber, which is capable of heating various gases to equilibrium temperatures in excess of 12,000"F (6,649"C). The arc heater is operated by tan- gentially injecting a gas into the arc chamber and then striking an arc between the electrodes. The injected gas is heated as it passes through the region occupied by the

Table 2. High-Temperature Screening Facilities

Performance Characteristics Subsonic Supersonic Subsonic Electric Supersonic Electric Oxy-Acetylene Torch Oxy-Kerosene Torch Arc-Jet (250 kw) Arc-Jet (250 kw)

Gas Temperature, O F , max. 5,700 Max. ("C) (3,149)

5,400 (2,982)

Gas Enthalpy, B tu l Ib 1,600 1,500 1,000 to 10,000 2,500 to 12,000 (cal/g) (888) (833) (555 to 5,555) (1,389 to 6,666)

Gas Velocity, f t l sec 100 to 300 4,500 1,000 to 3,000 6,000 to 12,000 (m/ sec) (31 to 91) (1,372) (305 to 914) (1,829 to 3,658)

0.005 to 0.040 (2.3 to 18.1)

0.005 to 0.040 Gas Mass Flow, Ib/sec 0.002 to 0.021 0.05 [glsec) (.91 to 9.5) (22.7) (2.3 to 18.1)

0.1 to 1.5 (kglcmz) (<1) (>1) (1 to 2) (0.1 to 1.6)

Cold-Wall Heat-Flux, Btu/ftz-sec) 25 t o 700 200 to 850 100 to 1,500 10 to 800 (call cm2-sec) (6.8 to 190) (54 to 231) (27 to 407) (2.7 to 217)

Stagnation pressure, atm <1 >1 1 to 2

Composition of Test Medium Combustion Combustion Air, N2, Air, N2 Products, Products, A, He, A, He Oxidizing, Oxidizing, coz, 02, Reducing or Neutral Reducing or Neutral co

Available Exposure Time, sec Near Continuous Near Continuous 30 to 600 30 to 600

Maximum Specimen Diameter in. 1.5 2.0 1.0 1.5 (mm) (38.1) (50.8) (25.4) (38.1)

~

240 SPE TRANSACTIONS, OCTOBER, 1963

Figure 2. Subsonic a i r arc-jet facility.

electrical arc discharge. It is then exhausted through an annular, water-cooled electrode into a plenum chamber and mixed thoroughly. The hot gas is finally discharged through a nozzle as a uniform high-temperature stream. Test specimens are prepared in the form of nominal 1-in. diameter rods, flat plates, wedges, pipes or some other geometric shape required to obtain the proper surface heating rate and flow conditions. To maintain the initial specimen shape during ablation, care must be taken to match the specimen geometry and size with the test medium. The specimen is exposed to one-sided heating in the plasma flame for a period of 15 to 300 seconds, during which time the instrumentation necessary is actuated to measure the material's behavior. The specimen is then withdrawn from the gas stream and rapidly cooled. The general appearance of several different plastic specimens after arc jet exposure is shown in Figure 3.

A vacuum chamber may be attached to the arc heater, and the plasma stream exhausted into the reduced pressure chamber. With this experimental arrangement, it is possible to obtain supersonic and hypersonic gas velocities, higher gas enthalpies, or simulated high altitude test conditions.

Specialized Environmental Facilities Since it is not always possible to obtain all of the de-

sired environmental conditions simultaneously, it is frequently necessary to evaluate promising plastics further in other test facilities. One or more critical environmental parameters are generated by these specialized test facilities, or perhaps they offer some other unique capability such as long testing times or large diameter test environments.

Pebble Bed Heater-The pebble bed heater offers a means for obtaining supersonic, high-pressure air in which erosion-resistant plastics may be further evaluated (26, 27). This type of facility is illustrated in Figure 4, and its general performance characteristics in Table 3. The facility is operated by first preheating the ceramic bed with hydro- carbon combustion gases. The heated gases are passed down through the vertical bed of zirconia (and possibly alumina) spheres for a period of time necessary to reach the desired


Figure 3. Plastic models af ter supersonic a i r arc-jet exposure. A. Teflon, B. Phenolic Nylon and C. Polyurethone

temperature. The burner is then turned off, and the combustion products are purged from the hot bed. High pressure air is introduced at the bottom of the columnar bed. As the air rises through the heated pebble bed, its temperature is raised to the desired value. At the top of the chamber, the hot air is led through a water-cooled nozzle where it is either exhausted into the open atmosphere or into a vacuum chamber. An instrumented test specimen with a nominal YZ to 2-in. diameter and a 3-in. length is placed in the exposure site about one inch from the nozzle end. Evaluation consists of rapidly moving the hemispherical or flat-faced model into the gas stream for 10 to 20 sec and quickly removing it.

Since supersonic air is used as the ablative medium, the experimental results are directly applicable to environments involving high speed aerodynamic flight. Another advantage of this type of facility is the high mass-flow, high pressure air which is available to determine the susceptibility of

241

ablative plastics to high mechanical pressure and shear forces. The inherent limitations of the pebble bed heater are: a slight decrease in air temperature with exposure time, slight contamination of the air (less than %%) from the zirconia pebbles, and tendency of the zirconia spheres to rise in the bed with a high mass-flow of air.

Graphite Resistor Furnace-The unique advantages of this facility are the high-temperature, high-pressure gas stream, which can be provided with considerable economy, versatility, and controllability (28, 29, 30) . The limitations of the electrically heated furnace are centered on the properties of the furnace material and the rate of heat transfer from the furnace walls to the gas stream.

Large Rocket Motor-The exhaust products of large rocket motors (31, 3 2 ) and jet engines (33) have long been used for evaluating large ablative bodies in supersonic, high pressure gas streams. Families of motors are available for this purpose, with a relatively wide range of hyper- environmental conditions. These include: heat fluxes from 100 to 2,500 Btu/ft2-sec (27 to 678 cal/cm2-sec), subsonic to Mach 3.5 (1.2 km/sec) gas velocities, stagnation pressures up to 15 atmospheres (15.5 kgf/cm2) and gas temperatures up to 8,800"F (4,87loC), for test area diam-

eters ranging from 2 to 15 in. (5.1 to 38.1 cm) . The desired environmental exposure conditions are obtained by proper choice of the test facility, fuel-oxidant mixture, specimen geometric configuration, and exposure position. For example, the desired surface heating rate to a flat plate can be varied by changing the exposure angle. For geometric models, the specimens may be moved forward or backward in the exhaust to achieve similar results. This latter technique is also useful for programming variable heating rates with time, but attendant changes in other environmental parameters occur with distance from the nozzle (such as a stagnation pressure drop with distance from the nozzle). If the surface heating rate is not a major consideration, and supersonic flow is used, the specimen should be located in the first shock diamond to prevent shock-boundary layer interactions.

Test specimens are prepared in the form of flat plates, wedges, hemispheres, cones, or possibly more complex shapes. The specimen is firmly attached to a fixed support, a moveable arm for raising and lowering the model in the flow, or a rotating wheel and exposed successively in the flame. If the first procedure is utilized, a removable heat shield is placed between the specimen and the nozzle. The

Table 3. Specialized Environmental Facilities

WATER-COOLED NOZZ Performance Pebble Bed Graphite Resistor large H202-Kerosene

Characteristics Heater Furnace Rocket Motor OIL BURNER

CERAMIC SWERES Gas Temperature, "F 1,900 to 4,000 4,600"F Max. 3,800 ("C) (1,038 to 2,204) (2,538) (2,093)

W I C BRICK Gas Enthalpy, B tu l lb 1,200 1,300 1,100 (ca I/ g) (667) (722) (611)

Gas Velocity, f t l sec 3,600 to 5,100 3,000 6,700 (mlsec) (1,181 to 1,673) (984) (2,198)

Gas Mass-Flow, lbisec 0.1 to 10 0.01 to 0.06 - Figure 4. Schematic of a pebble bed heater. (g/sec) (45 to 4,536) (4.5 to 27) -

Stagnation pressure, atm 7.0 Max. 2.7 Max. 9 (kg/cm2( (7.2) (2.8) (9.3)

Cold-Wall Heat Flux 50 to 400 160 to 750 900 Max. Btu/ftZ-sec (14 to 108) (43 to 203) (244)

(Cali cm2-sec)

Combustion Products

Composition of Test Medium Air Nitrogen

Available 50 to 300 60 Exposure Time, sec

30

Maximum Specimen Diameter, in. (mm)

0.75 0.50 12 (19) (13) (305)

242

-~ ~


motor is then ignited and stabilized. Detection and recording instrumentation are actuated for subsequent measurement of the materials response. The heat shield is then rapidly withdrawn and specimen exposure commences. After a period of 5 to 60 sec, the test period is terminated by re- placing the heat shield in front of the exposed specimen. Specimens on a rotating wheel are exposed in succession until all materials have been subjected to the high-temperature stream.

High-Temperature Simulation Facilities Ultimately, it is necessary to evaluate the most promis-

ing materials in environmental simulation facilities to obtain the data necessary for optimum design. Due to the limitations of available facilities, it is frequently impossible to obtain all of the desired environmental parameters simultaneously (34) . The problem thus resolves itself to a selection of a facility that most nearly simulates the intended application environment. For re-entry environments, the facility selected will likely be an arc wind tunnel or possibly the wave superheater (soon to be operational). For liquid propellant applications, either a gaseous or liquid subscale motor would be appropriate. For solid propellant applications, only the subscale solid propellant motor or possibly the gaseous propellant with particle injec- tion will satisfy the requirements.

The basic purpose of a simulation facility is to obtain design data on materials intended for use in a specific environment. It is necessary, therefore, to have close control over the environmental parameters, and be able to measure all of the important material indices of performance. The costs incurred in satisfying these requirements are generally high, i.e., $250.00 per specimen or higher. For this reason, the number of materials evaluated in a highly specialized simulation facility will be tempered primarily by economic considerations.

Arc Wind Tunnel-The arc wind tunnel is the only available materials and structures test facility, which is capable of closely simulating re-entry conditions. It has therefore been used extensively to obtain the design data required for ballistic, orbital, and glide re-entry heat shields (35 to 39).

The arc wind tunnel is basically an electric arc jet, which exhausts into a reduced pressure chamber. Consequently, it is capable of providing low surface heating rates from high enthalpy gas streams. The construction of the arc wind tunnel is akin to the previously described electric arc-jet, except for the high degree of refinement which is necessary for accurate test work. This degree of facility sophistication is evident in Figure 5, a photograph of a supersonic (Mach-4) (1.37 km/sec) air arc wind tunnel.

There are significant differences in the aerothermodynamic simulation capabilities of available electric-arc wind tunnels. The environmental characteristics of an operational supersonic air arc wind tunnel are given in Table 4. Certain environmental parameters have been maximized in other facilities, such as enthalpies to 19,000 Btu/lb (10,555 cal/g) (40) , stagnation pressures to 20 atmospheres (20.7 kgf/cm2) (41), and expanded flow velocities to about Mach 6 to 8 (2.1 to 2.7 km/scc) (42). With continued improvements in electrode design and the use of magneto- hydrodynamics, it is anticipated that considerably higher pressures and lower air contamination levels will be achieved. Gas velocities will likely increase slowly, due to the limitations of high performance vacuum pumps required to maintain very low tunnel pressures (43).


Figure 5. Supersonic air arc wind tunnel.

Current limitations on arc wind tunnels do not permit simultaneous achievement of all re-entry environment parameters, nor their variation with time. The experimental approach followed is to duplicate the surface pressure and enthalpy of the free flight situation, and evaluate the ablative models under these conditions. The effects of environmental parameters not correctly duplicated (such as heat flux) must be determined by subsequent independent study. Using this simulation technique, the heat of ablation value and most of the material-environment interactions at the stagnation point of a subscale model are expected to be representative of the free flight laminar boundary layer application. Away from the stagnation point, the lower gas velocities in the supersonic tunnels permit the shock wave to stand off farther from the model. The gas flow adjacent to the material surface is of a lower velocity, which per- mits a nonsimulated (excessive) amount of oxygen to diffuse to the ablating surface (40) .

Wave Superheater-The aerothermodynamic aspects of hypersonic re-entry into the earth's atmosphere are best simulated with a shock tube, but, unfortunately, exposure times are on the order of milliseconds. Consequently, the shock tube is unsuitable for determining the ablation characteristics of materials and structures. Recently, a new type of shock tube facility has become operational which prom- ises to greatly extend the running times. It is known as a wave superheater (44) with a capability to produce an uncontaminated flow of air at very high temperatures, hy- pervelocities, high pressures, and long times for realistic hypersonic testing in the laboratory. Anticipated environmental parameters of the facility are given in Table 4.

The wave superheater is composed of many shock tubes, which are mounted on the periphery of a rotating drum. Each shock tube is charged and fired sequentially in rapid succession to produce a continuous jet of uncontaminated, superheated gas. The gas in turn is collected and expanded through a nozzle and test section. The ablative model is positioned in the test section, which has a maximum diameter of eight inches.

It is anticipated that the wave superheater will be used increasingly for the evaluation of ablative plastics and

243

Figure 6. Schematic of a gaseous hydrogen-oxygen rocket motor.

structures in closely simulated hypersonic flow conditions. Subscale Gaseous Propellant Motor-The gaseous hydro-

gen motor (45, 46) is one of the most versatile facilities for evaluating plastics in rocket exhausts. It generates the temperatures, velocities and combustion products of interest (47, 4 8 ) , which are further defined in Table 4.

A representative hydrogen-oxygen motor is schematically illustrated in Figure 6. The motor is of conventional design, with suitable instrumentation for monitoring and closely controlling the operating parameters. It may be equipped with a particle injector, which is used for introducing various metallic powders into the gas stream to simulate impact and erosive effects of solid propellant gases. It may also have inlet ports for introducing reactive gases into the flow to simulate high-temperature corrosive environments.

When sufficient test material is available, the entire subscale nozzle is fabricated from it. The performance of the material in the entrance, throat, and exit section of the nozzle is thus obtained. If the test material is in short sup- ply, it is characterized in the throat section only. This is accomplished by fabricating a throat section and then adhesively bonding it in a nozzle housing of another standard reinforced plastic material. The test specimen is bolted to the aft end of the motor combustion chamber, and evaluated by exhausting the hot gaseous products through it. Con- tact of the reactive combustion products with the exposed surfaces produces material ablation and throat erosion. Thermal degradation is most pronounced in the throat section, since this area is subjected to the most intense heating and forces. As the throat section erodes and its diameter enlarges, the motor chamber pressure decreases accordingly. When a predetermined amount of throat ablation has taken place, the motor is either manually or automatically shut off. If the primary interest is in evaluating materials for the entrance or exit section of a nozzle, a water-cooled throat is used in the experimental setup. Longer firing times are thus achieved, which enable steady- state ablation information to be obtained. A cross-sectional photograph of a plastic nozzle before and after firing is shown in Figure 7.

Subscale Solid Propellant Motor-A family of solid propellant motors has been developed for the evaluation of ablative plastic materials, which are intended for use in rocket exhausts. The environmental conditions generated by these motors vary greatly in accordance with the composition and amount of propellant used and the particular motor design. Careful consideration of each of these factors will permit the proper selection of a test facility to

Table 4. High-Temperature Simulation Facilities

Performance Arc Wind Tunnel Wave Superheater Subscale 6eseous Subscale Solid Characteristics (250 kw) Propellant Motor Propellant Motor

Gas Temperature, "F - 8.500 2,000 to 6,300 4,700 to 6,640 ("C) - (4,704) (1,093 to 3,482) (2,593 to 3,6711

Gas Enthalpy, Btu/Ib 1,300 to 7,400 6,000 1,600 1,700 (cal/g) (722 to 4,110) (3,333) (889) (944)

Gas Velocity, f t i sec Supersonic 9,000 to 23,000 3,800 to 8,800 3,000 to 7,700 (mlsec) (2,743 to 7,010) (1,158 to 2,682) (914 to 2,347)

Gas Mass-Flow, Ib/sec 0.0005 to 0.0026 4tO 12 (glsec) (0.2 to 1.2) (1,814 to 5,443)

0.3 to 0.4 (136 to 181)

0.6 to 1.2 (272 to 544)

Stagnation or 0.34 to 1.0 Chamber Pressure atm (0.35 to 1.0)

195 (201)

3 to 55 (3 to 57)

6 to 100 (6 to 103)

(kg/ cmz)

(cal/cmz-sec) (0.8 to 27.1) - (2711 (15 to 51.5) Cold-Wall Heat Flux Btu/ftz-sec 3 to 100 - 1,000 Max. 55 to 190

Composition of Test Medium

Air Air Combustion Products, Combustion Products, Slightly Reducing with/without A1203

Available Exposure Time, sec 30 to 1,000 15 Max. 3 to 200 15 to 60

Maximum Specimen Diameter, in. 1.5 (mm) (38.1)

8 (203)

3 (762)

5 (127)


Figure 7. Subscale plastic nozzle specimens before ond after exposure.

closely simulate a specific rocket motor environment. Table 4 lists some of the environmental test conditions which have been achieved in available subscale solid propellant motors (49 to 51).

Rocket test motors utilize either a core-burning or end- burning propellant. The latter propellant type appears to offer greater flexibility, since the firing time can be varied by simply changing the grain length. The desired motor chamber pressure is obtained by varying the propellant burning rate or the nozzle throat area. The desired flame temperature and gas composition are obtained by proper selection of the propellant composition. The two major dis- advantages of subscale solid propellant motors are their high costs and inability to terminate the firing once the propellant is burning even though the test specimen may have already failed. Consequently, it is frequently impossible to conduct post-exposure examination of failed specimens.

Rigid ablative plastics intended for use as nozzle inserts, expansion cones, jetevators, blast deflectors, etc., are prepared in an appropriate specimen form and evaluated by exposure to a solid propellant flame. Many of these compo- nent test materials can be evaluated simultaneously, thus leading to greater economy of test. For example, a candidate plastic is fabricated into a subscale nozzle specimen and attached to the end of the motor. Evaluation is car- ried out by exhausting the combustion products through the nozzle. Meanwhile, candidate insulative plastics are being exposed in the upstream motor tube section. In the down- stream position outside of the nozzle, test jetevator bars or vanes are also being exposed. Discretion must be used with this experimental procedure, however, since the ablation of one test specimen may influence the performance of another. To illustrate, if excessive vaporization of a nozzle entrance insert occurs, increased cooling of the nozzle throat may result causing decrease in the erosion rate.

Flexible ablative plastics intended for motor case or nozzle insulation are evaluated under less severe test conditions. Case insulators are evaluated as a cylindrical blast tube which is attached to the rocket motor. Up to eight different materials are arranged circumferentially to form the test blast tube section; evaluation is accomplished by

passing through it the high temperature combustion products. The appearance of the specimens after firing is shown in Figure 8. The differences in materials’ behavior with re- spect to linear erosion, depth of char, swelling, uniformity of ablation, may be seen readily.

Aft motor insulation is evaluated on the convergent face of the nozzle. Six test panels may be individually embedded in the nozzle inlet section and exposed simultaneously. After firing, the specimens are removed for further observation and measurements.

Figure 8. Rocket motor blast tube with eight exposed plastic insulative materials.


Nonsimulation Laboratory Devices and Procedures Radiant heating methods are useful in basic studies of

ablative and pyrolytic behavior of plastics because of the following inherent attributes: no chemical interaction between the heating medium and the material, no mechanical removal of material by shear or pressure forces of a flowing gas stream, opportunity to collect gaseous products of decomposition, opportunity to provide a controlled environment (vacuum, air, or inert gas) for the material being heated, opportunity to vary the heat flux over a wide range from maximum to very low levels by simple energy at- tenuation methods.

The principle of operation of a radiant energy imaging furnace is the collection of radiant energy from an emitting source (such as the sun, a tungsten filament, or carbon arc) and the concentration of this energy at a single plane (exposure site) by suitable mirrors or lenses (52). The discussion here will be limited to those devices employing carbon arc energy sources, since these have been found to be the most versatile and reliable in ablation studies.

The two most common designs of carbon arc image furnaces are the double mirror type and the quartz lens condenser relay type. The double mirror type provides higher heat flux than the quartz lens type, but over a smaller area, and with less uniformity. In a commercially available “air- pinched’ double mirror carbon arc furnace, heat fluxes as high as 800 Btu/ft2-sec (217 cal/cm2-sec) are achieved over an area 0.25-in. (6.3 mm) in diameter. The Gaussian distribution of energy in an outward radial direction from the center of the spot resembles a probability curve having a peak at the center of the spot. The energy distribution can be made more uniform by placing a “flux redistributor” between the imaging mirror and the specimen surface (53). The flux redistributor is a hollow metallic pipe having a reflective inner surface, and of square or other noncircular geometric cross-section. The quartz lens condenser relay type provides reasonably uniform heat fluxes as high as 100 Btu/ft2-sec (27 cal/cm2-sec) over an area 1-in. (25.4 mm) in diameter. In both types of furnaces, the energy can be easily attenuated by metal screens having appropriate mesh size. In some models, the heat flux range can be changed from high to low by selection of appropriate electrodes and by reducing amperage to the electrodes.

Heat flux is readily measured with a metal foil type water-cooled radiometer, which furnishes a millivolt output from a thermocouple junction formed by the metal foil (constantan) and a copper block. Such radiometers must be precalibrated against a reference standard.

In exposing materials to the high intensity thermal radiation for specific time periods, it is not practical to start and shut off the arc for each exposure. Several different types of electrically operated shutter systems have been designed to control the exposure duration within less than 0.1 sec.

In studying heat dissipation characteristics of plastics, the sample is usually in the form of a flat slab which is heated at one surface. The heat flux to the surface is measured calorimetrically. The sample is exposed for a ccrtain duration and loss in weight is determined. The thermal efficiency is then calculated as total energy input divided by loss in weight of material. When radiant heating is used, there is no mass transfer cooling, and the heat dissipation is the sum of energy of decomposition, energy stored in the material and energy radiated from the surface (54, 55). The values of thermal efficiency obtained in radiant heating exposures may not necessarily correlate with ablation effi-

ciency, since there is no flow in the environment. For example, silicone laminates show a high thermal efficiency under radiant heating, but may perform poorly in some ablation environments since they are low in mechanical strength. However, in some ablative environments where strength and adhesion are not a factor, silicone resins performed well.

Furnace Pyrolyses-Information on the molecular weight and chemical identity of the gaseous products of plastic decomposition is of interest both from the standpoint of engineering analysis and the study of decomposition mechanisms. In radiant heating exposures, the collection of gases is accomplished by exposing the specimen in a quartz tube connected by a valve to a sampling bottle. The gases collected are then fed to a mass spectrometer for deter- mination of molecular weights. For the various molecular weights, applicable chemical formulas can be assigned, such as for hydrogen, water, methane, etc. In a few cases, there is some ambiguity, such as an identical molecular weight of 28 for carbon monoxide and nitrogen.

The principal uncertainty concerning the validity of the pyrolytic gas analysis studies is whether the final cooled gaseous products are the same as the initial pyrolytic products. One method of studying this factor involves the use of a “Time-of-Flight” mass spectrometer, which is aug- mented with a hot zone for keeping the gases at the decomposition temperature during analysis. In operation, electrically neutral gas molecules enter the mass spectrometer through a pinhole leak. They are immediately ionized, and a complete mass spectrum is generated once every 100 milliseconds. This fast analysis tends to minimize the appearance of secondary products of reaction.

Thermogravimetric Analysis and Differential Thermal Analysis-In both thermogravimetric analysis and differential thermal analysis, plastic samples are heated at a preselected rate of temperature rise through and beyond their decomposition temperatures. In TGA, the data output is in the form of a graph showing residual weight fraction versus temperature. In DTA, the data output is in the form of a graph which indicates the temperatures at which energy is liberated from or absorbed by the plastic, and the relative magnitude of these energy changes.

Electroless Discharge Apparatus-In re-entry environments the hot gaseous air plasma surrounding the ablative plastic is expected to be highly chemically reactive due to its dissociation into atomic species. One approach to studying the chemical reactivity of atomic oxygen and nitrogen with plastics in a non-hyperthermal environment is to ex- pose plastic samples to gases which are dissociated by a radio frequency discharge (54). The data obtained are weight loss, identity of gaseous products of degradation and visual or photographically magnified observation of the surface.

MEASUREMENT OF MATERIALS RESPONSE Behavior and performance characteristics of ablative

plastics during high-temperature exposure are obtained by qualitative observations and quantitative measurements. External and internal specimen instrumentation is used to obtain the necessary data before, during, and after material exposure.

Dynamic Measurements Specimen Immersed in Test Medium-Macroscopic sur-

face reactions and profile specimen changes during high- temperature exposure are obtained by direct observation.


Various deteriorative processes are noted, such as material spalling, flaking, delamination, particle shear, liquid run-off, vaporization, boiling, and similar effects. Gross changes in specimen geometry may also be observed.

High-speed motion pictures provide a more convenient and permanent record of ablative materials behavior. This technique has several additional advantages, which include the ability to restudy the recorded data, to observe minor surface reactions by film enlargement, and to make profile measurements from the film. Minor problems may be encountered in photographing the specimen surface during material ablation, however, due to the presence of a hot gas stream, pyrolytic gaseous products, and surface radiation. For example, the intense radiation from arc heated air (and possibly the specimen surface) will necessitate the use of an appropriate filter and film for good contrast and definition.

The linear ablation rate at any instant during material exposure is obtained by a photographic technique, electrical switching involving insulated breakwire gauges (61), or thermocouple burnout. A photographic technique for measuring surface recession with time is illustrated in Figure 9. Six 35 mm, sideview silhouette photographs are shown with a fixed position reference stud and an ablating phenolic-silica surface. The total surface recession is measured and plotted as a function of exposure time. From this plot, both transient and quasi-steady state linear ablation data are obtained. Transient ablation is that initial exposure period in which the surfacc recession (ablation) rate varies with time. Quasi-steady state ablation is denoted by a nonvariant rate of surface recession, which may be quickly determined from the straight line portion of the ablation curve. If optical photographic coverage is difficult to obtain during laboratory exposure tests, one must resort to breakwire gauges, thermocouple burnout techniques, or x-ray photography for measuring the rate of linear ablation.

Transient and quasi-steady state ablation temperatures are determined by optical radiation pyrometry, which in- cludes monochromatic, bichromatic and total radiation techniques (62). Monochromatic instruments measure the surface radiation intensity over a narrow spectral band (usually about 0.650 microns), and yield an optical “brightness” temperature. This value can be converted to yield a true surface temperature. If the surface temperature is steady- state, a manually operated instrument will be satisfactory. Transient surface temperature measurements, however, will require the use of an automatic brightness pyrometer (63). This latter instrument alternately scans a reference radiation source and the ablating surface at many cycles per second, and records the amplified emf output. The optical brightness temperature can be converted to a true surface temperature using Planck‘s law, provided the emittance value is known. Since the surface emittances of ablating plastics are not generally known, this experimental procedure has limited utility. Another technique for obtaining surface brightness temperatures is based on the use of a monochromatic infrared spectrometer and an appropriate mirror system (64). The experimental arrangement is shown in Figure 10. The intensity of surface radiation over a specified region (usually 2 to 15 microns or less) is recorded, and then compared to the spectral radiation characteristics of a black body. In Figure 11, black body emittance curves are given for various temperatures along with an experimental curve for ablating polyethylene. The surface brightness temperature is obtained by locating the black body curve

0 SEC 5 SEC

00 15 SEC 20 SEC

10 SEC

30 SEC

Figure 9. Silhouette photographs of an ablating phenolic-silica specimen used to determine linear ablation rate.

which lies just above (or touches) the experimental curve. At a wavelength of 3.4 microns, the approximate brightness temperature is 760°K (908°F). Bichromatic (two-color) pyrometers bypass the surface emittance problem by measuring the surface radiation intensities at two different spectral wavelengths. The resultant energy ratio may then be correlated with the temperature, using Wien’s law. It is assumed that the surface emittance values are constant at the selected wavelengths. Any departure from this as- sumption is reflected in the accuracy of the temperature measurement (65, 66).

Many techniques are available for measuring total radiation from ablating plastic surfaces, two of which shall be described. The first technique employs a thin metal foil radiometer, which is used to measure the thermal flux from the ablated specimen immediately following exposure (67). The fast-response radiometer is mounted on a swinging

ELECTRIC \ / ARC AIR

HEATER

q q ABLATING SAMPLE OR

REFERENCE BLACKBODY

OPTICS

LIGHT CHOPPER

SPECTROMETER

Figure 10. Schematic of an apparatus for measuring the radiation intensity and characteristics from an ablating plastic surface.

SPF TRANSACTIONS, OCTOBFR, 1963 247

arm, away from the specimen being exposed. At the moment of exposure termination, it is automatically positioned in front of the ablated specimen. The heat flux from the hot ablated surface is thus measured continuously as a function of time. This cooling curve is then extrapolated back to the instant of specimen exposure termination. Accurate surface radiation data are obtained by taking into account the following correction factors: angular aperture of the radiometer, initial error in the cooling curve due to a time lag, and convective heating of the radiometer by tail-off of the hot gases from the test facility. A second total radiation measuring system (23) has been devised in which measurements may be made continuously during material exposure. The experimental system is composed of a series of mirrors for collecting the surface radiation and refocus- ing it on a calibrated, single or multiple junction thermopile. The empirical arrangement is similar to that shown in Figure 10, except that the recording spectrometer is re- placed with an “Eppley” thermopile. Radiation emanating from the ablating surface is collected at a given angle to the specimen. Care is taken to insurc that the surface remains flat during exposure, and that the specimen image com- pletely covers the sensing element of the thermopile. The total surface radiation is then computed from the experimental data using Lambert’s cosine law. The accuracy of the radiation values will depend upon such considerations as: possible deviations of the angular surface radiation from Lambert’s cosine law, surface irregularities, and possible radiation effects from the hot gas stream, ablative gaseous products and other stray radiation.

The ability of a plastic material to restrict high temperatures to the exposed surface region is generally measured in terms of an insulation index or an effective thermal diffusivity. Each performance index is based on empirical temperature data, which is obtained in the material substrate during surface heating conditions. The insulation index is generally expressed in terms of a minimum thickness (or weight) of material necessary to maintain a preselected backwall temperature at the end of a heating period. The experimental procedure involves placing one thermocouple at the specimen backface, and possibly a series of thermocouples at varied positions (in depth) from the specimen surface. The plastic specimen is then x-rayed to determine the exact position of the metallic thermocouples. The emf output of each thermocouple is recorded continuously throughout the test, and for a short period following exposure. These data are then used to determine the time-dependent rise in temperature at given substrate positions. They may also be used to construct temperature profile curves in the material at specified exposure times. The thermal insulative characteristic of an ablating plastic may also be expressed in terms of its thermal diffusivity value. This empirical value is an inverse expression of the time required for heat to diffuse through a material. By definition,

(1) k

CDP (Ye = -

where a, is the effective thermal diffusivity, k is the thermal conductivity, C, is the specific heat and p is the material density. The thermal diffusivity of non-carbonizing, homogeneous plastics, such as polyethylene and polytetrafluoro- ethylene, can be calculated with reasonable accuracy from individual property data. Ablative composite plastics, however, form numerous discreet reaction zones in the material

substrate, which have greatly dissimilar chemical and physical properties. Complex heat and mass transfer processes occur in these reaction zones, thus greatly distorting the temperature distribution. For this complex case, it is most appropriate to compute a procedural thermal diffusivity value (14) in accordance with the following relation:

where (Y, is the procedural thermal dausivity, V is the linear ablation rate, T, is a temperature at position x, Tb is the temperature of the intact nonheated material, and (dT/dT) is the slope of the temperature versus time curve at position x. Test conditions of steady-state ablation and unidirectional heating are assumed. The accuracy of the insulation results are dependent, then, upon the assump- tions used in deriving the insulation indices, and the accuracy of the thermocouple data. For the later case, the thermocouple sensing errors can generally be held below 6% by: inserting the thermocouples parallel to the surface, insuring good contact between the thermocouple and the plastic material, and minimizing the amount of insula- tor and wire by using very fine diameter wire and polished thermocouple beads.

Structural properties of reinforced plastics have been determined during actual ablation conditions (28, 68, 69). A standard tensile specimen is preloaded to a given per- centage of its ultimate strength, and then the necked-down portion is exposed to the high-temperature environment. Unidirectional flow of heat to one side of the specimen produces material ablation. The specimen ultimately rup- tures and the two disjoined parts are automatically pulled out of the high-temperature environment. Measurement of the specimen load throughout the exposure period and at time of rupture yields its load bearing capacity. These values must be adjusted by calibration runs, since material ablation results in unsymmetrical loading of the specimen.

Specimen Surrounds Test Medium-The measurement of a material’s response is greatly complicated when the test specimen surrounds the high-temperature environment. This case is typified by the evaluation of a subscale rocket nozzle in which the hot combustion products are exhausted through the test material.

Many different techniques have been employed for in- direct measurement of the linear rate of ablation. A convenient means for determining the instantaneous and total erosion in the nozzle throat is based on a continuous monitoring of the rocket chamber pressure. As the throat material is ablated and its diameter increases, the chamber pressure falls proportionally. Since the chamber pressure is related to the total throat area, a correlation chart can be prepared to express these variables. Consulting this chart quickly determines the amount of throat ablation. The symmetry of throat ablation, however, cannot be determined by this procedure. Thermocouple burnout offers a second technique for sensing the linear ablation rate. High- temperature thermocouples are placed at known depths in the material substrate. Common practice is to drill the necessary holes into the nozzle wall, insert the thermocouples, and then tightly repack the walls with the drilled material. The thermocouples may also be ceramic insulated and adhesively bonded in place. A great disadvantage of this procedure is that the thermocouple sensor frequently affects the local ablation processes. Failure occurs first in this region, often with catastrophic results. This problem

SPF TRANSACTIONS, OCTOBER, 1963 248

has been largely overcome with the use of a preformed plastic plug in which the thermocouple is located (70). The plug material is identical to that of the test plastic nozzle, otherwise dissimilar material ablation rates may lead to serious errors. The thermocouple plug is threaded into the nozzle wall prior to test. The loss of surface material during ablation ultimately exposes the thermocouple wire. The moment of thermocouple destruction is thus obtained from the emf output data, which are taken continuously during the exposure period. A similar technique involves the use of breakwire gauges, which are based on electrical switching. This type of plug sensor consists of a series of metal wire switches, which are located at known, preselected depths from the surface. With surface ablation, the wire switch is ultimately exposed to the hot gas stream. It then undergoes melting and the electrical switch is opened. An attendant voltage change corresponding to that sensor loss is recorded. The electrical signal thus indicates the instantaneous position of the ablation front. Other less widely used techniques for measuring the linear rate of ablation are based on the use of ultrasonics, radioactive absorption or backscatter and microwaves.

Since the ablating surface cannot be viewed directly during exposure, its surface temperature is difficult to obtain. Small diameter metallic filaments having known melting temperatures may be embedded parallel and at varying depths to the surface. The filaments are then examined for possible melting after material exposure, by means of visual, microscopic, x-ray, and metallographic techniques. A second procedure for estimating the surface temperature utilizes temperature-position data obtained during the exposure. A series of thermocouples are used to measure simultaneously substrate temperatures at known positions. These data are then used to construct a curve of temperature versus substrate depth. Extrapolation of this curve

back to zero depth (specimen surface) fves the approximate surface temperature. The principa difficulties with this procedure are the usual exponential shape of the temperature curve, and the inability to obtain accurate temperature values near the high-temperature ablating surface. The penetration of heat into a material substrate during exposure is measured by thermocouples which have already been discussed.

Post-Exposure Measurements. In view of the many technical difficulties which are encountered in obtaining dynamic materials performance data, it is imperative that additional post-exposure measurements be made on the residual ablated specimen. Table 5 summarizes the more conventional measurements which are performed, the technique of measurement, and the type of information obtained. Such information can be used to furnish additional insights on the more subtle phenomena and mechanisms of ablation, as well as what actually took place during the ablation process.

Summary Evaluation of ablative materials performance is accom-

plished by exposure to convective and radiant heating devices having varying degrees of simulation fidelity to the anticipated design environment. In exposing plastics in hyperthermal environments, it is extremely important to characterize the environmental parameters of enthalpy, stagnation pressure, shear force, and heat flux, and to use a specimen configuration readily amenable to performance analysis.

Valuable data may be obtained during materials exposure including information such as rate of surface recession, surface temperature, heat re-radiated from the surface, and internal temperature distribution. For materials which f6rm

Table 5. Post-Exposure Measurements on Ablated Plastic Specimens

Type of Measurement

Profile changes Surface roughness

Chemical composition

Density

Porosity and pore spectra

Surface area

Microstructure

Compressive strength Hardness

X-ray reflection

Technique

Silhouette photography micrometers Optical profilometer Brush analyzer Elemental chemical analysis

Volume displacement (beads) Mercury displacement Mercury intrusion under pressure

Nitrogen adsorption

Resin impregnation and photomicrography Micro-structural test Rockwell hardness test

X-ray reflection

Information Obtained

Total linear changes Possible localized spalling

Presence of newly formed compounds Composition of surface residue Secondary pyrolysis, evidence of Most volatile components in composites Mass per unit volume of discrete reaction zones Open and close cell porosity Pore size and distribution Degree of surface reactivity Presence of very small diameter pores Cell wall structure Zones of reaction Stress-strain curve Resistance to ball indentation and percent recovery Degree of crystallinity Identification of newly formed compounds


a char layer, additional information such as mechanical properties, microstructure, and chemical composition may be obtained from post-exposure laboratory analysis.

Caution should be exercised in extrapolating ablation performance data from a given set of environmental conditions to other conditions. The validity of such extrapoia- tion depends on the chemical and physical mode of ablation of the material and on proper application of the rela- tionships of heat transfer and mass transfer cooling.

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THE END


Documents

Evaluation methods for ablative plastics. A review of the techniques used