Desarrollos recientes en materiales elastoméricos para su uso como blindaje térmico en motores cohete y perspectivas futuras

La posibilidad de que se produzca una ruptura catastrófica durante el vuelo de cohetes y misiles exige una investigación focalizada en el aislamiento de los motores de cohetes. En la actualidad, la optimización de polímeros ablativos como aislamiento para el tubo sólida del motor cohete tiene un papel determinante en el éxito de estos.

Catastrophic breakdown that occurs during the flight of supersonic vehicles demands more focused research in the insulation of rocket engines. At present, optimization of polymeric ablatives as viable insulation for solid rocket motor casing has a prominent role in the successful mission of rockets. Among polymers, elastomer serves an imperative part. Comprehensive investigations were disclosed, especially in the elastomeric heat shielding materials with various reinforcing agents. In this paper, research progress of mostly used elastomers is reviewed, and a circumstantial understanding about the features of ablation and insulation has been validated.

1 INTRODUCTION

Space industry plays an important role in a country’s economic and national level prestige in the military and civilian purposes or the designing of forthcoming proposals to launch space craft into outer space. Progress in the science and technology made enormous contributions that illuminated the successful transportation of material things surpassing the earth’s atmosphere. Rockets deliver payloads for scrutinizing the assigned defensive or investigative space missions. Every aircraft entering into any planetary surface at hypersonic speeds can cause severe aerodynamic heating (around 2000‐3000 K1) on the outside of the vehicle due to the burning of solid propellant.24 Therefore, to protect the vehicle from miserable failure of spacecraft, some design features are added to cope with the aerodynamic heating.5Thermal protection system (TPS) materials impart heat shield to protect the structure, aerodynamic surface and the payload of missiles, and those vehicles interacting with the hypersonic environment.67

Polymeric ablative materials serve to effect as TPS that defends space vehicles and probes during the atmospheric entry; they are effective to produce passively cooled rocket combustion chambers8 and to render insulation to the solid rocket motors (SRMs) at high temperature.911 Although some nonpolymeric materials such as inorganic polymers/ceramics or metals have been used as ablative TPS, polymeric ablatives represent the most flexible category of ablatives. In contrast to the inorganic polymers, polymer ablatives have more inborn benefits: high‐heat shock resistance, lower density, good mechanical strength, and thermal insulation capabilities.12

The broad range of heat shielding materials (HSMs) for rocket motor casing is produced from reinforced thermosets or elastomers.1319 The most assuring material for the HSM for rocket motors is the elastomers, which can be produced as rubbers, foamed rubbers, or fiber‐reinforced composite materials. On the grounds of lightweight, low thermal conductivity, stability in the mechanical and thermal stresses during the operation, elastomeric HSMs (EHSMs) can find relevance in the insulation of SRM casing where thermal insulation is a prime necessity. They do not possess the ability to form cokes without the introduction of special additives due to the linear chain structure of elastomers, thus no solid carbonic residue on thermal decomposition. Also, elastomers have high elongation at break and the capacity to withstand the mechanical and the thermal stresses during the manufacturing and in use.20

This review puts the elastomeric materials as ablative insulators for SRM into frame and would confer the readers the progress that has been attained in developing relatively low surface eroding and ablative insulating systems for SRM. These systems can be used to manufacture variety of ablative TPS systems for rocket motor casing application and would provide an idea regarding the composite system that would behave elastomeric as well as ablative. Scrutinizing these systems would pursue the researchers to take the advantage of elastomeric materials for rocket motor casing insulation application.

2 BROADNESS TO SRM INSULATION

Solid rocket motors are still used today because of its simplicity and reliable launching features.21 Among the 2 types of basic systems, cartridge‐loaded systems are used for defense applications and the case‐bonded ones for the longer range and higher payload service capacity.22 Solid rocket motor consists of an outer casing with the purpose to accommodate the propellant grains. Solid rocket motor case is usually fabricated from metals such as steel or carbon/epoxy filament wound composite.810 Figure 1 represents the component parts of an SRM. To be a good casing component, it needs to satisfy various requisites both thermally and mechanically.

  • It must have outstanding bonding with the propellant and outer motor covering throughout the whole working temperature range of rocket motor.
  • Low ablation rate (0.09 to 0.2 mm/s) and low density to lower the dormant mass of the thrust unit.
  • Low thermal conductivity in the range of 4 to 5 × 10−4 Cal/cm/°C and high specific heat in the range of 0.4 to 0.5 Cal/g/°C.
  • Formation of porous char with smaller dimensional change and good adhesion.
  • Able to tolerate mechanical and thermal stress during the operation and handling of the components.
  • Low moisture absorption with good aging behavior (minimum of 10 y of shelf life).24
  • Minimize the specific gravity to <1.15 g/cm3 for increasing its firing range.

Figure 1
Solid rocket motor component parts23 [Colour figure can be viewed at wileyonlinelibrary.com]

The casing‐enclosed propellant is systematized to endure combustion, and as a result of that, it achieves the needed thrust for acquiring the rocket propulsion.2526 Thermal environment in the combustion chamber will be above 3000 K, and the radiation process attained by the combustion products may affect the structural integrity of the SRM. Consequently, internal ablative insulation is provided for shielding the rocket motor from the combustion of propellant grains.27 A seam of heat barrier material positioned between the internal surfaces of the outer protective covering of an SRM and the propellant can be marked out to be the internal insulation for the solid rocket casing.28 The cardinal task of internal insulation is to avoid the rocket motor casing from reaching high temperatures that may risk the structural virtue of the outer casing28 (Figure 2 represents the cross‐sectional view of SRM outer casing). Also, interior insulation of the rocket motor casing also performs several auxiliary roles: (1) It hinders the burning on the propellant piece surfaces, where the combustion is objectionable; (2) it acts as a shock‐absorbing piston during the conveyance of case stretch into the propellant; (3) it controls the transfer of chemical species within the motor; (4) the impact of combustion products over the case can be obstructed; and (5) it encloses the case, joints, and fittings to avoid the fall in pressure and ruin from the hot burnt ones and (6) for pointing of the burnt products into the nozzle with the maximum limit that could be attainable.29

Cross‐sectional view of a solid rocket motor outer casing [Colour figure can be viewed at wileyonlinelibrary.com]

3 ABLATIVE INSULATION: A NECESSITY

Ablative materials are outlined for anticipating the thermal protection to the systemic parts exposed in drastic thermal environments.8 Therefore, those materials can have a significant arena in aerospace technology. Along with the exterior parts of missiles and rockets, they have a prominent role in the insulation of SRM. Ablatives are capable to consume high heat, good heat allocation, better insulation, lightweight, good resilience, easy to find, cheap, and can be easily designed for a particular purpose.24 These materials are superior insulators, because comparatively low amount of heat gets transferred to the rocket motor structure when ablatives are manipulated as insulators. Therefore, focusing on the ablative composites for HSM in SRM can be a most effective choice of insulating materials.

High temperature–resistant metals or alloys that are used in the rocket motor components alone cannot outlast during the working conditions of the motor. Metal component parts are necessary to provide skeleton of a rocket motor. Along with that, ablative materials are essential for thermal safeguard of the rocket motor. Due to its low thermal conductivity, specialized burning and natural evolution of vigorous char on the exterior of insulator by the automated heat and mass transference.30 Thermal derogation of ablatives and the decadence of the culminated volatiles consume most of the pyrolysis heat.31

Ablation reaction triggers by the virgin material pyrolysis. It can be observed in Figure 3 that as the pyrolysis reaction advances, volume of virgin material lowers and char volume increases. Selection of proper ablative material relies on the hyperthermal environment (temperature, pressure, shear rate, and conditions of the atmosphere) in which the TPS is needed to function.9 There is no single material that satisfies all the operational conditions in a successful manner. Therefore, depending upon the prerequisites, many polymeric ablatives have been developed during the last 5 decades.33 Various physicomechancial properties that control the ablative performance in a composite system were represented in Table 1. Solid residue remained after pyrolysis, gaseous by‐products, elemental composition, heat conductivity and heat capacity, permeability of char, and duration of time lasting in the surface of the specimen can be the significant parameters, holding the ablation behavior of a composite.

Schematic representation of the ablation process32 [Colour figure can be viewed at wileyonlinelibrary.com]
Table 1. Component properties in a composite controlling the ablative behavior34
Components Properties
Polymer 1. Elemental composition2. Molecular mass

3. Degree of cross‐linking

4. Tg

5. Thermal decomposition temperature

6. Thermophysical and thermodynamic properties

Catalyst or curing agent 1. Elemental composition2. Percentage retained in polymer
Reinforcing agent and fillers 1. Elemental configuration2. Materialistic nature

3. Alignment in composite

4. Thermal decomposition temperature

5. Thermophysical and thermodynamic properties

Ablative materials 1. Solid residue2. Gaseous products

3. Elemental composition

4. Mechanical properties

5. Permeability and distribution of pores

6. Heat conductivity and specific heat capacity

7. Duration in residue

Composite 1. Uniformity of material distribution2. Presence of defects, voids, etc

Polymeric materials can be used over a wide range of hyperthermal conditions,35 and as long as the pyrolysis starts, these materials get easily removed from the surface of derogation, following the thermal degradation of the insulation layer of SRM.9 Accordingly, numerous kinds of polymeric ablatives are conventionally used for SRM insulation.

4 POLYMERIC MATERIALS AS ABLATIVES IN THERMAL PROTECTION: AN OVERVIEW

In contrast to metals and ceramics that were used in the earlier days of rocket motor insulation, polymeric materials have amassed inherent advantages. Metals mislay their potency at high temperature. Although ceramics excel metals and polymers in their melting temperature, strength, and thermal expansion properties, their brittleness limits their use as structural materials. Advancements in the field of polymers along with the materialization of strong and stiff reinforcements like carbon, silica, and aramid fibers assist to meet the requirements of modern aircraft.23

Since polymers can be tuned to different high‐temperature environments, polymeric ablatives demonstrate the most adaptable class of HSMs. Different types of polymeric ablatives can be summarized in Figure 4. They can be fiber‐reinforced polymeric ablatives, rigid HSM and EHSM, lightweight ceramic ablators, and nanostructured polymeric ablatives. Ablators with greater charring effects are the predominant ones for thermal insulation in spacecrafts. Over the last 40 years, high char holding phenolic resins have been used in the production of numerous fiber‐stiffened polymeric ablatives.36 But silica/phenolic composites acquired with antioxidizing features are the most prevailing ablators.830

Classification of polymeric ablatives12 [Colour figure can be viewed at wileyonlinelibrary.com]

Thorough‐going exploration in polymeric ablatives initiated at the time of rivalry between the Union of Soviet Socialist Republics and the United States during the 1950s. MX‐4926 (a composite of carbon fiber of woven rayon and carbon black filler in phenolic matrix) developed by the Cytec Engineered Materials is the most famed material for the standardization of the National Aeronautics and Space Administration and other research federations.37 Patton et al38 relied upon the MX‐4926, which is also a phenolic‐based composite for the fabrication of polymeric ablatives for their study with inexpensive and readily available vapor‐grown carbon fibers. High charring phenolic resin probably be the most vulnerable polymeric material of all the time. Si et al34 prepared phenolic resin incorporated graphene oxide, bestowed with excellent thermal resistance attributed by the good interfacial interaction arising between graphene oxide and phenolic resin inducing the formation of high char environment. Bahramian39 incorporated graphite into phenolic matrix, thereby enhancing the thermal properties of the composite with the additional graphite layer formed on the surface of the nanocomposite.

Tate et al,37 Mirzapour et al,40 Srebrenkoska,41 and Srikanth et al42 prepared and evaluated the carbon‐phenolic composite, and Bahramian et al32 improvise it again with the addition of graphite and kaolinite. Pulci et al43 demonstrated the effective manufacturing of carbon‐phenolic composite for reentry space vehicles. Ding et al44 improved the carbon‐phenolic composite with the introduction of zirconium silicide particles. Chen et al45improved the ablation performance with the use of zirconium diboride particles in carbon‐phenolic matrix. Park et al,46 Srikanth et al,47 and Yum et al48 prepared the carbon‐phenolic composite with the implementation of carbon nanotube (CNT), whereas Natali et al49 compare the ablative properties of carbon black–reinforced phenolic composites with MWNT/phenolic composites. While Robert et al50 investigated about the silica‐phenolic composite mutated with nanoclay, Paydayesh et al51 evaluated about the organoclay‐modified phenolic resin matrix. Wang et al52 and Winya et al53 focused on the incorporation of phenolic resin with nano‐aluminum oxide and glass fiber. All those composites that were prepared according to the aforementioned inclusion of different fillers and reinforcements are being exploited in the rocket motor insulation.

As a high‐performance resin, epoxy has a popular trademark in the development of polymeric ablatives for insulator rocket motor casing. Yu et al,54 Ahmad et al,55 and Puglia et al56 furnished epoxy‐based composites and reported about their thermal conductivities. Similarly, Firouzmanesh et al57 prepared carbon/epoxy novolac composite as leading candidates in SRM insulation with high thermal resistivity and good ablation properties, using the high char retention nature of phenolic resin along with the superlative nature of epoxy. Such type of systems was initially remarked by G. J. Fleming.58 Even though, polymeric materials are presently handled as insulating materials for SRM, researches are buttoned up to target on the efforts to replace polymers with elastomeric ablative chemical compositions for insulating rocket motor casing applications. The capability of elastomers to get adapted to the deformations under the impact of mechanical‐thermal stresses can be the reason for relying on elastomers as HSM for rocket motors.

5 ABLATION IN ELASTOMERS

Ablation process empowers the structural materials to sustain and bestow safety in harsh environments. It can be defined as a well‐organized heat and mass transfer action where large quantity of heat energy gets scattered during a short period with typical loss of material. It is a complicated procedure encompassing phase changes like melting vaporization, sublimation, and pyrolysis.5960 Ablative composites follow a complex mechanism for providing the thermal safeguard that incorporates heat rejection through reradiation, action of heat generated from the deterioration of the materials, insulation, enormous heat absorption, and the latent heat of thermal deterioration. Ablating elastomeric shielding uses up the heat generated from the propellant pyrolysis gases with the coalescence of enthalpy of heat changes (heat needed to augment the temperature of pyrolyzed species from atmosphere to temperature of propellant ignition gas).9

When the ablative materials decompose, it yields pyrolysis gases at the reaction site that deteriorate forming a char blanket at higher temperatures. The existence of char layer controls the entry of heat into the surface, and it produces an abrupt temperature gradient.61 Those pyrolytic species get trapped in the boundary layer; enthalpy gets lowered, undergoing thermochemical reaction. The ablation process can be followed up by a scheme of reactions represented in Figure 5. The ablated organic species upgrade to a surface char covering, which removes the thermally ambiguous polymer from the extremely huge temperature atmosphere. This surface char layer consumes heat by endothermic processes, provides heat capacity at extreme temperature environment, and adds up secondary endothermic reactions with the liberation of hydrocarbon gases or residual reinforcing agent.

Mechanism of ablation process [Colour figure can be viewed at wileyonlinelibrary.com]

High surface ejection with low heating rate at the material surface exposed in extreme temperature can disperse an enormous amount of heat via surface radiant emission. Newly formed char will persist with the substrate for a small interval, and a deteriorated layer gets generated below the char layer, where pyrolysis of virgin material occurs. Gases in the deterioration zone percolate with the pressure that enhance the thermochemical reactions to pyrolytic carbon.62

If the casing insulation material is not reinforced, the altogether stress developed in the char layer by the pyrolysis gases and thermal expansion causes deteriorated material to escape away from the surface.61 During the further progress of heating zone, mechanical erosion of the metallic particles in the propellant along with the hot exhaust gases will take place. When the surface layer erodes, the next layer gets vulnerable, and so on the process continues. Large amount of endothermic heat engrossed by the pyrolysis reaction and the porous char layer formed at the surface offers outstanding conservation to the underlying material.63 Amidst of different polymeric ablatives seen in Figure 4, elastomeric ablatives represent the most tractile heat shield. Nowadays, elastomers can be the most assuring HSM for SRM.

6 A FOCUS ON EHSMS FOR SRM

An HSM needed to be tolerant to act elastic to the anamorphosis concealed by the mechanical and thermal loadings bared during the fabrication and working of the shielding material.81064 In any puncture happening to the insulation material during the operation of SRM, the casing will be susceptible to melting or degradation, and it leads to the permanent failure of rocket motor. Consequently, EHSMs are accounted to insulate the inner surface of rocket engines. It can withstand the temperature up to 2800°C and pressure 100 bar (inside the rocket motor).10 There are a lot of important requirements that need to satisfy along with the basic necessities for the preparation of an EHSM, making the formulation task of elastomeric insulation more complicated.

Most elastomers can achieve better thermal stability, mechanical strength, improved insulation, better charring process, and ablation insulation performance with the inclusion of chopped fibers or fibrous materials into it.65 Donskoy suggested that assimilated fillers and the nature of vulcanizing system will alter the competency of HSM.66 Some elastomers such as ethylene propylene diene monomer (EPDM) and dimethyl silicone rubber have least specific gravity, which break down endothermically and form a char covering in the course of ablation; however, uncompounded elastomers have poor resistance to surface erosion and processing properties. Hence, elastomers are coupled with additives and inorganic fillers for incorporating specific processing attributes, principally to boost the ablation resistance.96768 Elastomeric materials such as nitrile rubber,69 silicone rubber,7075thermoplastic polyurethane (TPU),6376 and ethylene propylene diene rubber7786 are mostly used for making an insulation casing for rocket motor cases.

6.1 Nitrile rubber–based HSMs

Nitrile rubber (Perbunan, Buna‐N), a synthetic copolymer of acrylonitrile and butadiene,87 is used in the development of HSM for rocket motor casing.8206688 Heat shielding materials based on nitrile rubber are on vogue in Indian space program.89 The physical and chemical properties of nitrile rubber depend upon the composition of nitrile content, and it can be resistant to oil and fuels. However, it is able to tolerate a range of temperature from −40°C to108°C90; processing of nitrile butadiene rubber (NBR) is probably difficult, due to the strong intermolecular interaction. It can be cross‐linked by sulfur or peroxides and vulcanized at temperatures around 140 to 190°C.12 Nevertheless counteracted with the limited shelf life, high density (1‐1.3 g/cm3), and low‐temperature properties, NBR is still used as an insulation liner for SRM.649195 Heat shielding material made of NBR usually contains SiO2 as a powder and asbestos fibers939596 for reinforcement and thereby to improve the thermal stability. The aim of NBR use as EHSMs is because of their greater affinity with higher char yield resins such as phenolics, which are added for the betterment of ablation resistance.97

A notable archetype of NBR‐based HSM was Hitca 6520,66 used to shield steel case of Titan III‐C SRM.64 Heat shielding material developed by Rock Island Arsenal Company was composed of phenolic resin and asbestos fibers.98 The elasticity of the modified composite was lost after the incorporation of phenolic resin.64 GTR‐V‐44, GTR‐V‐45, and Garlock 7765 are some commercially available HSMs made of NBRs with the inclusion of silicon dioxide and asbestos as fillers.20 Zhao et al65 prepared the chopped polyimide (PI) fiber–filled NBR insulation for SRM by the surface modification of chopped PI fibers under oxygen plasma and thereafter by mixing it with NBR in a 2‐roll mill. It showed that the excellent mechanical and ablative properties of the NBR contribute to the high thermal stability of the SRM. Ablation rate decreases with the increase of PI fiber content. Plasma treatment can efficiently enhance the roughness of PI fibers and can form many polar groups on the surfaces, which improvise the interfacial interactions between PI fibers and the NBR matrix. In comparison to the Kevlar‐filled NBR, chopped PI‐filled NBR shows better insulation. By this method, Zhao et al conclude that this current approach provides better charring and ablative properties as a HSM.

Iqbal97 group prepared a composite of NBR with different concentrations of phenol formaldehyde using internal dispersion kneader and 2‐roll mixing mill. It was observed that ablation resistance of the composite becomes great and the thermal conduction, shore A hardness, tensile strength got increased with increasing the phenol formaldehyde in the rubber matrix. Guan et al67 conducted a study about the ablative properties of the hydrogenated nitrile rubber using fillers such as organically modified montmorillonite and expanded graphite, compromising many polar nitrile groups and reported that rise in ablation rate and a decline in the linear ablation rate of MMT‐reinforced HNBR composite compared to silica or expanded graphite.

6.2 EPDM‐based HSMs

Generally, insulation used in SRM is composed of organic rubber such as an aramid fiber–filled EPDM.99 Ethylene propylene diene monomer rubber is a terpolymer of ethylene, propylene, and a diene component (M includes the rubbers having a saturated main chain of methylene group). Figure 6 indicates the chemical structure of EPDM elastomer. Commonly used dienes for EPDM are dicyclopentadiene, ethylidenenorbornene, and vinyl norbornene.67 According to the necessary application of the EPDM, ethylene, propylene, and diene ratio can be varied.100 Ethylene propylene diene monomer elastomers have been fabricated by using vanadium‐based Zeigler‐Natta catalyst systems.101 The reliable mechanical properties like tear strength, immense thermal resistance, and good retention of properties after aging due to the saturated main chain make EPDM rubber as good insulators. Since EPDM is a material with minimal specific gravity, its novel application is in the case‐bonded insulation for rocket motors.102103 Also, it has been proven to show better aging properties,81104108 and it possess high filler loading capacity.104 Thermal stability of EPDM is analogous or greater than NBR.109

Structure of ethylene propylene diene monomer

Saturated main chain is the reason behind the thermal stability of EPDM.110 It has reliable low‐temperature properties: Tg of −50°C, permitting them to be used in different environmental conditions.109 It has well‐known resistance to oxidation, ozonization, weathering changes, and superior low‐temperature properties. It also has the lowest density among the elastomers approximately 0.85 g/cm3.111 This lower density has a prominent significance in the reduced weight of HSM. Therefore, such kind of elastomers can be used for SRM with high payload capacity.226684 In addition to the above mentioned properties, the weight of the launching vehicle and satellite performs a massive role in rocket flight. Table 2 gives the data about the specific gravity of different elastomeric ablators. Therefore, researches are concentrated to find a proper substitute for nitrile rubber (specific gravity of 0.97‐1.00) with EPDM rubber (specific gravity of 0.80‐0.87).105

Table 2. Specific gravity of different elastomeric ablators130
Different Elastomeric Ablators Specific Gravity
Dimethyl RTV, low density system 0.87
Phenyl‐methyl RTV 1.40
All methyl elastomers 1.17
All phenyl‐methyl elastomers 1.57

Ethylene propylene diene monomer is used as both primary and secondary polymer to enhance the bonding properties; otherwise, it will give lesser bonding characteristics. Addition of high levels of fibrous reinforcement to elastomeric matrix can regulate the mechanical properties of the HSM.6985112 Ethylene propylene diene monomer with carbon fiber,113 aramid fiber,111 silica, PI,81 and hydroxyl‐terminated polybutadiene114has been used profitably in rocket motor insulation. Along with that various additives such as fillers, plasticizers, activators, accelerators, curatives, antioxidants, tackifiers, and flame retardants are involved in a typical EPDM insulation formulation.84 Fillers such as carbon fiber, silica, asbestos, cork, and glass fiber can be used for improving the ablative efficiency.115 Some of the EPDM composites that are used as insulating material for rocket motor casing are listed in the Table 3.

Table 3. Some EPDM formulations existed for rocket motor casing insulator
Sl no. EPDM Composite Formulation Observation Reference
1 EPDM + asbestos fiber/Fe2O3/cork powder + curing agents + additives Decrease in mass loss and erosion and an improvement in the heat of ablation due to the increase in fiber loading. 115
2 EPDM + epoxy + curing agents + additives Resistance to ablation can be better improved with higher melting point substrates and by enhancing the matrix‐substrate interaction. 82
3 EPDM + chlorosulfonated polyethylene + HTPB + silica powder + napthenic oil + sulfur‐thiuram Incorporation of polar CSE to the rubber matrix will improve the interfacial bonding between insulator and propellant. HTPB facilitates the processing and peel strength of EPDM rubber. Amount of HTPB added influenced the interfacial bonding levels of nonpolar EPDM matrix. 114
4 Maleated EPDM + melamine fiber + resorcinol + hexa + precipitated silica + ZnO + Zn stearate Using the excellent heat insulating and flame resistant nature of melamine formaldehyde, showed improved insulating behavior of the composite 168
5 EPDM + liquid EPDM + PSA pulp + nanosilica + sulfur + ZnO + stearic acid + 2‐sulfonyl dibenzo thiazole + dipenyl guanidine Addition of 10 phr of pretreated polysulfonamide pulp to EPDM matrix, 3‐fold improvements in the ablative properties and a slight decrease in the tensile strength is seen. 169
6 EPDM + carbon black + DCP + stearic acid + ZnO + antioxidant Carbon black, surface area controls the cure kinetics of carbon black/EPDM composite. Sulfur at the surface of carbon black helps in EPDM composite vulcanization. 79
7 EPDM + liquid EPDM + nanosilica + additives + curing agents + PSA short fibers/aramid fiber Degradation peak of PSA fiber was 100° higher than that of aramid fiber. Increasing the fiber content in both the composite decrease the ablation rate initially and then increases. Ablation rate and thermal conductivity of PSA/EPDM composite is greater than aramid/EPDM composite. 170
8 EPDM + aramid pulp/kynol/silica short fibers + nanosilica EPDM/kynol gives out a char with smaller dimensional change and shows good adhesion on the virgin material. Insulation properties of EPDM/aramid composite are noteworthy. EPDM/silica showed poor performance. 111
9 EPDM + liquid EPDM + polysulfonamide pulp + nanosilica + epoxy phenolic resin + additives + curing agents Composite provides a strong char layer after pyrolysis. With 10 phr of epoxy phenolic resin, a 2‐fold increase in the ablative properties of the composite is observed. Also it promotes the interfacial bonding within the composite. 171
10 EPDM + surface modified polyimide + additives + DCP + sulfur Surface modified PI gives a rough surface and improves the adhesion between fiber and EPDM matrix. Thereby, showing superlative mechanical and ablative properties. 168
11 EPDM + benzoyl peroxide + maleic anhydride + ZnO + stearic acid + polyimide + carbon nanofiber + antioxidant + sulfur + accelerator Modification with maleic anhydride improves rubber‐filler compatibilization. PI enhances thermal stability and CNF improves mechanical properties and gives off better char yield. Maleic grafted EPDM/PI/CNF composite showed good physical and thermo‐mechanical properties. 80
12 EPDM + PI + nanosilica/nanoclay/carbon nanofiber + curatives Low density and enhanced mechanical properties acquired by the EPDM/PI/nanocomposite. EPDM/PI/nanosilica displayed superior retention of properties after aging. Composites of these fillers showed same maximum degradation temperature. 81
13 EPDM + silica short fibers + wollastonite + nanosilica + aramid fiber Makes only a partial replacement of asbestos fiber. Small size length of macron sized needles of wollastonite limits the adhesion between charred and pristine material. It is economical than aramid fibers. 86
  • Abbreviations: CNT, carbon nanotube; CSE, chlorosulphonated polyethylene; DCP, Dicumyl peroxide; EPDM, ethylene propylene diene monomer; HTPB, hydroxy terminated polybutadiene; PI, polyimide; PSA, polysulfonamide.

6.3 Silicon rubber–based HSMs

Silicon rubbers are linear polysiloxanes, which are distinguished according to the curing mechanism as high‐temperature vulcanized and room‐temperature vulcanized silicon rubbers.116117 Heat shielding materials based on silicone rubber ablative composites are an appealing material for SRM and ramjet propellant.118120 The exceptional thermal stability of silicon rubbers make it a reliable choice for higher temperature applications.70Lots of silicon rubber ablative composites for TPSs are known so far.121123 Polydimethyl siloxane, a familiar associative of this family, is stable to 300°C under vacuum.124 The siloxane bonds in silicon are highly stable and have high binding energy (106 kcal/mol) compared with carbon bonds (84.9 kcal/mol), and therefore, silicone rubbers have high thermal stability, chemical resistance, and have better electrical insulation properties than organic rubbers.125 The addition of methyl phenyl siloxane or diphenyl siloxane as a copolymer with polydimethyl siloxane has been proven to increase the degradation temperature to 400°C.126127

Polydimethyl siloxane is the most common silicon elastomer that is used as a matrix for SRM. At higher temperatures, it decomposes with the liberation of volatile compounds such as H20, CO2, and methanol.12 The carbonaceous char obtained after the pyrolysis contains mainly silicon compounds such as SiO2 and SiCO.128129 Silicon elastomers are considered as charring ablators. Depending on the nature of aerodynamic and vibrational forces acting on the charring material, char is either removed or remained on the ablator surface.130Heat shielding materials are prepared by blending silicone rubbers with fillers, fibers, and vulcanizing agents. Flame retardant additives such as antimony trioxide and polychlorinated compounds can be used.131 They have been practically applied as thermal protection components of Saturn V and Polaris rockets. Baldwin et al132 demonstrated the use of certain siloxane polymers containing a high percentage of vinyl methyl siloxane units as binders for ablative insulation for improving the ablation resistance in high‐temperature environment.

Silicon rubber composite comprising silica, SiC, and carbon fibers established by Dow Corning Corporation confronted a test flight successfully.133134 Sanden135 developed a higher heat‐resistant silicon‐based insulating material by mixing silicon rubber materials from Dow Corning and General Electric Company. Kim and his crew136 conducted a study about the thermal stability and ablation properties of the silicon rubber composites with clay and carbon fiber and found out that addition of carbon fibers may lower thermal stability of the composite. Torre and his coworkers investigated about the theoretical and experimental attributes of silicon rubber ablative composites.19137138

Yang et al139 prepared an ablative composite of silicone with zirconium carbide (ZrC) or zirconia (ZrO2), and it is practically certified as TPS. Thermal stabilities, tensile strength, and nature of ablation of the composite increased with the assimilation of ZrC or ZrO2 powders. Ablation rate was decreased by 40% and 72% by 40 phr ZrC and ZrO2, respectively. The ablated surface occupied with ZrO2, SiO2, and SiC acted as an effective hurdle for oxygen dispersal and temperature navigation, foiling additional decomposition and erosion, making it as an ablation resistant silicon rubber composite.

Yu et al140 evaluated the thermal constancy and ablation peculiarities of the aluminum silicate ceramic fiber with acicular wollastonite added in a silicon rubber composite and the TGA analysis confirmed that the thermal stability was enhanced by the incorporation of these fillers. A dense ceramic layer formed will act as an effective oxygen and heat barrier, liable for the increase in thermal stability and ablation properties. Liu et al141 investigated about the mechanical, ablation properties of silicon rubber composites incorporated with carbon fibers and observed that the added carbon fibers resist the stretching of rubber matrix. Linear ablation rate was also found to decrease with increasing the carbon fiber content.

Silicon‐based insulator, Room Temperature Vulcanized Silicon Rubber, RTV 560, is produced by loading silicon rubber with SiO2 and Fe2O3 particles. It decomposes by producing a foamed char containing silica, SiC, and FeSiO3.129 DC 93‐104,142143 another prominent silicon insulator, is composed of silicone elastomer loaded with silica, silicon carbide, and carbon fibers.144145 It has high viscosity, high char retention, and low ablation rate even under the domination of high shear stress environments20 that has been used as protective coating for rocket motor cases120 and ramjet combustion chambers.99146147 DC 93‐104 was modified with asbestos or PI fibers.148 Stephens et al149 patented for the use of aramid fibers in silicone‐based HSM.

Another significant silicone‐based TPS material is the super light ablator, eg, Lockheed Martin’s SLA‐561V.131137138 It was a low‐density material (0.25 g/cm3), initially developed for the Viking Lander missions by Martin Marietta Corporation in 1976.131 Because of its excellent mass and cost‐efficient performance, it has got recertification in the 1990s for Mars Pathfinder Lander (heat flux of 54 W/cm2) and Mars Exploration Rover (115 W/cm2). The ablating performance of HSM, SLA‐561V, is verified by testing under shear conditions, and it was applied as a thermal insulator for ramjet combustor.150 It worked as a charring insulator and showed medium surface erosion in this environment.151 In 2012, SLA‐561V fruitfully protected the Mars Science Laboratory (MSL) capsule during the reentry flight on Mars.152153 Table 4 compares the specific gravity of different elastomers, and it is seen that dimethyl RTV silicon rubbers have the least specific gravity, which is well suited for long fire range rocket motor casings.

Table 4. Ablation rates of different elastomeric HSM composites
Fiber/Filler Content in Elastomer Matrix, phr Linear Ablation Rate, mm/s Reference
1. Nitrile rubber–based HSM composite0 phr M. PI/NBR

2 phr M.PI/NBR

4 phr M.PI/NBR

6 phr M.PI/NBR

8 phr M.PI/NBR

6 phr Virgin polyimide/NBR

6 phr Virgin Kevlar/NBR

0.122

0.064

0.054

0.050

0.132

0.132

0.137

65
2. Nitrile rubber–based HSM compositeHNBR/fumed silica

HNBR/organically modified montmorillonite

HNBR/expanded graphite

0.063

0.047

0.067

67
3. EPDM‐based HSM compositeWithout PSA‐pulp fiber /EPDM

With 10 phr untreated PSA pulp/EPDM

With 10 phr pretreated PSA pulp/EPDM

0.03 ± 0.001

0.04 ± 0.001

0.01 ± 0.001

169
4. EPDM‐based HSM composite5 phr PSA/EPDM

10 phr PSA/EPDM

15 phr PSA/EPDM

20 phr PSA/EPDM

5 phr aramid/EPDM

10 phr aramid/EPDM

15 phr aramid/EPDM

0.015 ± 0.0001

0.01 ± 0.0001

0.02 ± 0.0001

0.04 ± 0.0001

0.02 ± 0.0001

0.017 ± 0.0001

0.03 ± 0.0001

170
5. EPDM‐based HSM composite0 phr EPR/EPDM

5 phr EPR/EPDM

10 phr EPR/EPDM

15 phr EPR/EPDM

20 phr EPR/EPDM

0.09

0.065

0.045

0.047

0.050

171
6. EPDM‐based HSM composite0 phr M.PI/EPDM

2.5 phr M.PI/EPDM

5 phr M.PI/EPDM

10 phr M.PI/EPDM

Parent PI/EPDM

Parent kevlar/EPDM

0.20

0.18

0.06

0.04

0.06

0.09

168
7. Silicon‐based HSM composite0 phr ZrC/silicon

0 phr ZrO2/silicon

10 phr ZrC/silicon

10 phr ZrO2/silicon

20 phr ZrC/silicon

20 phr ZrO2/silicon

30 phr ZrC/silicon

30 phr ZrO2/silicon

40 phr ZrC/silicon

40 phr ZrO2/silicon

0.075

0.075

0.0725

0.070

0.0675

0.065

0.055

0.045

0.045

0.02

139
8. Silicon rubber–based HSMRTV control

RTV/APP

RTV/ADP

RTV/OPS

RTV/OPS/APP

RTV/OPS/ADP

0.146 ± 0.004

0.122 ± 0.005

0.108 ± 0.001

0.121 ± 0.003

0.118 ± 0.004

0.079 ± 0.004

154
  • Abbreviations: ADP, aluminum diethylphosphinate; APP, ammonium polyphosphate; EPDM, ethylene propylene diene monomer; HNBR, hydrogenated nitrile butadiene rubber; HSM, heat shielding material; M.PI, modified polyimide; NBR, nitrile butadiene rubber; OPS, octaphenyl polyhedral oligomeric silsesquioxane; PI, polyimide; PSA, polysulfonamide.
  • Bolded figures denote the optimized composition from each EHSM.

Zhang et al154 synthesized silicon rubber matrix incorporated with ammonium polyphosphate, aluminium diethylphosphinate, and octaphenyl polyhedral oligomeric silesquioxane composite and make out a highly flame retardant, ablative silicon rubber composite. Addition of flame retardants systemically improved the flame retardancy, thermal stability, smoke suppression, and thereby the linear ablation rate.

6.4 Thermoplastic polyurethane–based HSMs

Thermoplastic polyurethane nanocomposites are recommended to substitute for the popular EPDM composites in the contemporary state of art TPSs.155 Thermoplastic polyurethane elastomer is created by the polyaddition reaction of polyol/long chain diol, a chain extender or short chain diol, and a diisocyanate component.156 Figure 7 indicates, the chemical structure of thermoplastic urethane formed from the polyol and diisocyanate in the presence of a chain extender. Thermoplastic polyurethane elastomer nanocomposites are the best choice of insulation material developed by Air Force Research Laboratory (AFRL) having lightweight, better ablation, good insulation features, and more cost effective fabricating method compared with the contemporary material, Kevlar‐filled EPDM.157

Structure of thermoplastic polyurethane [Colour figure can be viewed at wileyonlinelibrary.com]

Since CNTs are well‐known fillers owed to their large aspect ratio and outstanding mechanical, thermal, and electrical properties,157158 recently, Pircheraghi et al159prepared TPU/CNT composite with enhanced properties. In the soft segment of TPU, CNT gets well dispersed,160 confirmed by the ultraviolet absorbance.161 By regulating the sonication speed and filler dispersion, thermal conductivity of the obtained composite can be effectively reduced.

Jiao et al162 discussed about the collaborating flame decelerating and smoke annihilation effects of ferrous powder and ammonium polyphosphate in TPU matrix. Thermogravimetry (TG) and derivative TG results revealed that ferrous powder can minimize the decomposition temperature and can enhance the thermal stability and flame retardancy at larger temperatures. Lee et al163 studied the ablative properties of TPU elastomer with different nano reinforcements and concluded that multiwalled CNT composite is a good choice of ablative reinforcement, whereas carbon nanofiber is not an ideal ablative material. Jaramillo et al164 put an effort to optimize the formulation of TPU as an EHSM in an SRM along with other EHSMs.

Liu et al165 compare the mechanical and flame retardancy properties of the polyoxymethylene (POM) with melamine phosphate and TPU‐encapsulated melamine phosphate/POM. Analysis showed that POM/TPU‐wrapped melamine phosphate has the most preferable thermal resistance and least formaldehyde release rate, favoring to the future applications. Maucourt et al166 patented for hydroxyl‐terminated polybutaduiene/isophorone diisocyanate–based polyurethane composition for inner surface coating of propulsion unit. Allcorn et al56 investigated the potential of TPU elastomeric nanocomposite as HSMs and concluded that they can be effectively used as HSMs for SRM with the inclusion of the special additives and fillers.

Currently, Chen et al167 discussed about the synergistic effect of iron‐graphene and ammonium polyphosphate in TPU. Composite was prepared by solution‐blended master batch of TPU with iron‐graphene, finally melt blending of ammonium polyphosphate. Thermogravimetry and TG‐infrared data revealed that a highly thermally stable composite is obtained by this synergistic combination. All the ongoing research programs confer that TPU elastomer nanocomposites are an excellent choice of HSM in the nearby future.

7 A COMPARATIVE STUDY OF DIFFERENT EHSMS

Table 4 shows the ablation rates of different EHSMs observed in significant literatures causative to the insulation of SRM. The given data compare the ablation rates of various elastomeric composites at different fiber loading rate, pointing that optimizing the rate of suitable fiber loadings in an elastomer‐based HSM shows tremendous changes in the ablation rate. Studying the different composite ratios of significant elastomers, it is observable that manipulating the elastomers with different filler/fiber loadings in each composition (from Table 4) gets optimized in each formulation. An ideal ablating composite will be the composite with low ablation rate and high ablation resistance. Even though their specific formulation is optimized with low ablation rates for better thermal stability of SRM in harsh environments, their mechanical properties need to be satisfied for their future applications as HSM for rocket motor casing applications. While dealing with the specific mechanical parameters such as low specific gravity and better aging properties, EPDM can be the reliable option for EHSM in SRM.

8 CONCLUSION

In this review paper, various types of EHSMs were discussed as feasible candidates for SRM insulation and their fulfillment in practical applications. This review also includes the materials that could find application in the coming future, due to its properties like higher thermal stability and ablation characteristics. In the past decades, around 90% of heat shielding application in the SRMs were based on nitrile rubbers. Although space exploration missions beyond earth’s orbit are ready to equip with more temperature resistant and lightweight heat shielding ablative materials. The elastomers would be the better option among different polymeric ablatives with the figuration of suitable choice of fillers. The appropriate selection of fillers, reinforcements, and additives can modify the properties of shielding composite materials to a certain extent. Elastomeric composites fabricated with nanosized stiffening agents, advances to the new prototype for polymeric ablative materials.

Nowadays, elastomeric materials like silicon, EPDM, and TPU elastomeric composites provide greater contributions as TPSs in the hypersonic flight missions. In addition to it, the highly fabricated asbestos fibers as reinforcements in the EHSMs can be replaced with aramid fibers, CNTs, and with PI fibers. A composite with low ablation rate can dramatically influence the insulation properties at the same time; good mechanical properties of the composite is mandatory. Advancements in these materials can be obtained by adjusting the reinforcement concentration, incorporation of multiple additives, optimal fiber loading percentage, and suitable processing techniques. In conclusion, propagating need for HSMs in the rocket industry successfully can be satisfied by the EHSMs. They are the most reliable candidates to replace metals and ceramic ablatives in the rocket aerospace reentry missions. Hence, by upgrading to the new elastomeric formulation with vulnerable reinforcements such as aramid fibers and PI fibers, there would be a tremendous change in the arena of rocket motor insulation. Ethylene propylene diene monomer with low specific gravity and better aging characteristics would be the most vulnerable choice of elastomers in SRM insulation. Fabricating new EPDM composite formulations with pretreated fillers using the conventional flame retardants, antioxidants, and adaptable reinforcements, there would be a dramatical progress in the rocket motor industry. While considering the environmental impact of these materials, proper agenda must be bore to develop an environmentally sustainable EHSM formulation to meet the requirements of SRM insulation with the existing or newly framed reinforcements or fillers with EPDM. Therefore, the future of rocket motor industry lies with EPDM elastomers.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the R&D board (Grant Number: ARDB/01/1041841/M/1), Aeronautics wing of Defence Research and Development Organization (DRDO) for their support and funding.

Fuente: https://onlinelibrary.wiley.com