Joseph H. Koo

A Review of Numerical and Experimental Characterization of Thermal Protection Materials - Part III. Experimental Testing

Thermal protection materials are required to preserve structural components of space vehicles during the re-entry stage, missile launching systems, and solid rocket motors. A comprehensive literature survey was conducted to review the experimental test methods as well as erosion and heat transfer data relevant to the performance of thermal protection materials for different applications. Laboratory scale apparatus, small-scale and subscale solid rocket motors were used to test these thermal protection materials. A wide range of thermal protection materials such as cork, fiber-reinforced phenolics, fiber-reinforced silicone, filled-elastomers, filled-silicone nanocomposites, and phenolic fiber-reinforced nanocomposites were included. This paper is the third in a four-part comprehensive literature survey that is grouped into numerical modeling, properties characterization, experimental testing, and advanced nozzle throat material. I. Introduction hermal protection materials are required to protect structural components of space vehicles during the re-entry stage, missile launching systems, and solid rocket motors. A thorough literature survey was conducted to review the numerical and experimental characterization of these thermal protection materials for different military and aerospace applications. The literature survey is grouped into: (a) numerical modeling, 1 (b) materials thermophysical properties characterization, 2 (c) experimental testing, and (d) advanced nozzle throat material. In this paper, only the experimental testing literature will be discussed. The numerical modeling 1 and thermophysical properties 2 reviews were published previously, and advanced nozzle throat material will be presented in a subsequent review.

Flame Retardant Polyamide 11 and 12 Nanocomposites: Thermal and Flammability Properties

Polyamide (nylon) 11 (PA11) and 12 (PA12) were melt-blended, dispersing low concentrations of nanoparticles, namely nanoclays (NCs), carbon nanofibers (CNFs), and nanosilicas (NSs) via twin-screw extrusion. To enhance their thermal and flame-retardant (FR) properties, an intumescent FR additive was added to the mechanically superior NC and CNF PA11 formulations. For neat and nanoparticle-reinforced PA11 and PA12, as well as for PA11 reinforced by both intumescent FR and select nanoparticles (NC or CNF), decomposition and heat deflection temperatures were measured, as were the peak heat release rates while burning the composites. All PA11 polymer systems infused with both nanoparticles and FR additive had higher decomposition temperatures than those infused with solely FR additive. For the PA11/FR/NC polymer blends, only the 20 wt% FR and 7.5 wt% clay formulation passed the UL 94 V-0 requirement, while all PA11/FR/ CNF formulations passed UL 94 V-0 requirement.

Transient combustion in mobile gas-permeable propellants☆☆☆

Abstract Gas-permeable propellants possess great potential for producing sizable thrusts within extremely short time intervals. A research program was set up with the objectives: (1) to obtain a deeper understanding of high speed flame propagation and the gas dynamic behavior of two-phase reactive flow systems; and (2) to predict the rates of gasification during transient heterogeneous combustion of granular propellant beds. The following transient phenomena occur in a few milliseconds: penetration of hot gases into interstitial voids, convective heating of pellets to ignition, granular bed compaction and rapid pressurization from 1 to 2600 atm in a thick-walled steel chamber. Pressure transients and flame-front speeds are measured by high-frequency pressure transducers and ionization probes, respectively. A gaseous pyrogen ignition system utilizing a spark ignition was found most suitable for varying igniter strengths and achieving high reproducibility. The physico-chemical phenomena described above have been formulated into a theoretical model. The governing equations were derived in the form of coupled, non-linear, inhomogeneous partial differential equations. A stable, fast convergent numerical scheme has been used to solve these hyperbolic partial differential equations. The results show that the flame front accelerates significantly and the rate of pressurization increases substantially in the downstream direction. The igniter strength and the propellant gasification temperature were found to have pronounced effects on the pressurization process. The theoretical predictions are in close agreement with the experimental data.

Carbon/Phenolic Nanocomposites as Advanced Thermal Protection Material in Aerospace Applications

Ablative nanocomposites were prepared by incorporating multiwall carbon nanotubes (MWCNT) into phenolic resin and then impregnating them into rayon-based carbon fabric. MWCNT were blended into phenolic resin at 0.5, 1, and 2 wt% loadings using a combination of sonication and high shear mixing to insure uniform dispersion of MWCNT. The composite test specimens were tested by using an oxyacetylene test bed (OTB) applying a heat flux of 1000 W/cm2 for duration of 45 seconds. Composite specimens with 2 wt% MWCNT showed reduction in mass loss, recession in length, and in situ temperatures compared to control composites.

Flame-retardant polyamide 11 nanocomposites: further thermal and flammability studies

Polyamide (nylon) 11 (PA11) were melt-blended by dispersing low concentrations of nanoparticles (NPs), namely nanoclays (NCs) and carbon nanofibers (CNFs) via twin-screw extrusion. To enhance their thermal and flame retardant (FR) properties, an intumescent FR additive was added to the mechanically superior NC and CNF PA11 formulations. For neat and NP-reinforced PA11 as well as for PA11 reinforced by both intumescent FR and select NPs (NC or CNF), decomposition temperatures by TGA, flammability properties by UL 94, and cone calorimetry values were measured. All PA11 polymer systems infused with both NPs and FR additive had higher decomposition temperatures than those infused with solely FR additive. For the PA11/FR/NC polymer blends, Exolit® OP 1312 (FR2) is the preferred FR additive to pass the UL 94 V-0 requirement with 20 wt%. For the PA11/FR/CNF formulations, all Exolit® OP 1311 (FR1), OP 1312 (FR2), and OP 1230 (FR3) FR additives passed the UL 94 V-0 requirement with 20 wt%.

Thermoplastic polyurethane elastomer nanocomposites: morphology, thermophysical, and flammability properties

Novel materials based on nanotechnology creating nontraditional ablators are rapidly changing the technology base for thermal protection systems. Formulations with the addition of nanoclays and carbon nanofibers in a neat thermoplastic polyurethane elastomer (TPU) were melt-compounded using twin-screw extrusion. The TPU nanocomposites (TPUNs) are proposed to replace Kevlar-filled ethylene-propylene-diene-monomer rubber, the current state-of-the-art solid rocket motor internal insulation. Scanning electron microscopy analysis was conducted to study the char characteristics of the TPUNs at elevated temperatures. Specimens were examined to analyze the morphological microstructure during the pyrolysis reaction and in fully charred states. Thermophysical properties of density, specific heat capacity, thermal diffusivity, and thermal conductivity of the different TPUN compositions were determined. To identify dual usage of these novel materials, cone calorimetry was employed to study the flammability properties of these TPUNs.

Kinetics and Thermophysical Properties of Polymer Nanocomposites for Solid Rocket Motor Insulation

Thermal protection materials are required to protect structural components of space vehicles during the reentry stage, missile launching systems, and solid rocket motors. Novel materials based on nanotechnology creating nontraditional ablators are rapidly changing the technology base for thermal protection systems. In this study, different polymer nanocomposite compositions were created by melt-compounded montmorillonite nanoclays or carbon nanofibers in a neat thermoplastic polyurethane elastomer polymer using twin-screw extrusion. The kinetic and thermophysical properties that are required to analyze the ablation characteristics were measured for selective thermoplastic polyurethane materials. Properties of the nanomodified systems were then compared against those of the current state-of-the-art insulation material, Kevlar®-filled ethylene-propylene-diene monomer and the neat thermoplastic polyurethane elastomer, for the investigation of kinetic parameters using the isoconversion technique. Based on temperatures at peak weight-loss rate, Kevlar-filled ethylene-propylene-diene monomer outranked the proposed formulations at all heating rates. Recognizing that ablation performance is a complex function of kinetic and thermophysical/transport properties, a surrogate test for ablation performance was investigated. Samples of neat and nanomodified thermoplastic elastomer were run in an oxygen-consumption (cone) calorimeter. The peak heat release rates of the nanomodified samples were substantially less than that of the neat thermoplastic elastomer.

Flame-retardant Polyamide 11 and 12 Nanocomposites: Processing, Morphology, and Mechanical Properties

The objective of this study is to develop improved polyamide (nylon) 11 (PA11) and 12 (PA12) polymers with enhanced flame retardancy, thermal, and mechanical properties for selective laser sintering rapid manufacturing. PA11 and PA12 were melt-blended, dispersing low concentrations of nanoparticles, namely nanoclays (NCs), carbon nanofibers (CNFs), and nanosilicas (NSs) via twin-screw extrusion. To enhance their thermal and flammability properties, an intumescent flame retardant (FR) was added to the mechanically superior NC and CNF PA11 formulations. NC or CNF additions to either PA11 or PA12 generally increased its tensile strength and modulus, but sharply reduced its elongation at rupture. FR additives reduced PA11’s properties considerably. This substitution, however, only exacerbated the already steep drop in elongation at rupture due to FR additives alone; while elongation dropped 58% with the addition of 30 wt% FR, it dropped 98% with the addition of 25 wt% FR/5 wt% CNF.

Flammability Studies of a Novel Class of Thermoplastic Elastomer Nanocomposites

The thermal insulation properties of thermoplastic polyurethane elastomer nanocomposites were characterized at different heat fluxes. Thermoplastic polyurethane elastomer was modified with different loadings of montmorillonite nanoclays and carbon nanofibers (CNFs) via twin screw extrusion processing. The addition of nanoparticle into thermoplastic polyurethane elastomer resulted in the formation of a char layer and modified the thermal insulative properties of the material. It was found that thermoplastic polyurethane elastomer with 10 wt% CNFs and with 5 wt% nanoclays gave the best thermal performance with respect to protecting a substrate. The surface temperature of the thermoplastic polyurethane elastomer-clay nanocomposites did not vary much with addition of clay particles while the surface temperature of the thermoplastic polyurethane elastomer-CNF nanocomposites varied more substantially. Some of the trends in surface and substrate temperature measurements with nanomodification can be described using a simple energy balance model that takes into account the basic heat transfer mechanisms.

Theoretical Modeling of Intumescent Fire-Retardant Materials

A theoretical model is developed to predict the thermochemical be havior of intumescent fire-retardant coatings. The model is based on the assump tion that the intumescent reaction is analogous to the phase change process occurring over a finite temperature range. From the numerical results, it is found that the histories of the substrate temperature can be accurately predicted by choosing adequate pseudo latent heat and temperature range for the intumes cent reaction, and the bending evidence observed in experiments can be success fully predicted by the present intumescence model. Finally, it is shown that the present model can readily be extended to simulate the intumescent process with multi-intumescent zones.