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Polyamide 6 (PA6) was melt-blended with an intumescent flame retardant (FR), multi-wall carbon nanotubes (MWNTs), and nanoclay particles to produce multi-component FR-PA6 nanocomposites. FR-PA6 nanofibers were processed from varied nanocomposite formulations via electrospinning. Electrospinnability, morphology, along with combustion and thermal properties of the nanofibers were investigated. Both the bulk-form nanocomposites and the electrospun nanofiber membranes exhibited significantly improved combustion properties, including both Heat Release Rate and Total Heat Release. On the other hand, thermal stability appeared compromised. With proper FR additive concentrations, synergism between MWNTs and nanoclay was observed.

The introduction of inorganic nanomaterials as additives into polymer systems has resulted in polymer nanostructured materials exhibiting a multiplicity of high performance characteristics beyond what traditional polymer composites possess. These “multifunctional” features attributable to polymer nanocomposites consist of improved thermal and flame resistance, moisture and chemical resistance, decreased permeability, charge dissipation and conductivity. Through control/alteration of the additive at the nanoscale level, one is able to maximize property enhancement of selected polymer systems to meet or exceed the requirements of current commercial, military, and aerospace applications. The technical approach involves the introduction of nanoparticles into selected polymer matrix systems whereby some nanoparticles are surface-treated to provide hydrophobic characteristics and enhance their inclusion into the hydrophobic polymer matrix. The objective of this paper is to summarize our current research in using polymer nanocomposites for flammability properties and microstructure studies.

Nylon 11, a popular material for commercial use, has been combined with low-percent loads of carbon nanofibers (CNFs) to tailor mechanical, fire retardancy, and thermal properties. Transmission electron microscopy images show that the CNFs are randomly aligned in the polymer matrix. We show that the thermal conductivity is minimized at a certain percent loading of CNFs due to a large thermal contact resistance between the CNFs and the medium.

Polymer nanostructured materials are used as ablative and insulative materials in rocket propulsion system. This paper describes the progress of two ongoing research programs in developing new classes of nanostructured materials that are lighter and have better erosion and insulation characteristics than current ablatives. For rocket nozzle ablative, Borden Chemical’s SC-1008, a resole phenolic, was selected as the resin. For the rocket motor internal insulation, Dow’s thermoplastic polyurethane elastomer and Dow Corning’s thermoplastic polyamide elastomer were selected as the base polymers. Several nanoparticles such as montmorillonite (MMT) nanoclays, carbon nanofibers (CNFs), and polyhedral oligomeric silsesquioxane (POSS®) have been evaluated with the SC-1008 phenolic resin and the thermoplastic elastomers. Wide angle X-ray diffraction and transmission electron microscopy were used as screening tools to determine the degree of dispersion before producing materials for testing. Cytec Engineered Materials fabricated several MX4926 alternates by replacing the carbon black filler with selected nanoparticles. Several candidates showed good performance improvements in erosion rate and heat-soaked temperature rise over MX-4926 (baseline).

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

A number of existing and developmental advanced heatshield materials were screen-tested at Boeing’s Space System Laboratory (SSL) Graphite Heater Test Facility to evaluate their thermal performance. The simulated thermal environment was 60 BTU/ft2-set for 90 seconds at ambient pressure followed by 160 seconds of thermal soaking. Based on the primary constituents, these low temperature ablative (LTA) materials were grouped into cork-based and silicone foam materials. The cork-based materials with epoxy or phenolic resin smoked and combusted during the high heat environment and swelled significantly as they charred. The combustion of cork and resin to form weakened char was the single most important degradation path of such heatshield materials. The finely cracked top surface is a characteristic of all cork based materials and was observed on most posttest samples. One of our new cork phenolic materials produced rigid char and ablated smooth. Replacing the epoxy and phenolic resins with silicone also minimized the combustion issues. Additionally, the silicone foam material produced smooth rigid char and the ablation rate was approximately equal to the swelling rate, resulting in minimal shape change. The main disadvantages of silicone-based thermal protection system (TPS) materials are the relatively high cost and more involved installation process. *Engineering Specialist tAUA member, Sr. Engineering Specialist

A transient, 2D material erosion model is developed to describe the simultaneous processes of thermochemical ablation and mechanical erosion of ablative materials. The model predicts the rate of surface recession and the erosion pattern by considering the effects of (1) thermal loading (i.e., applied surface heat flux and duration of heating), (2) particle loading, (3) transient heat conduction in the ablative material, (4) conversion of the kinetic energy of the impacting particles into thermal energy, (5) thermochemical degradation or pyrolysis of the polymer matrix, (6) advection due to the flow of decomposition gases, (7) thermal expansion of the carbonaceous char-layer, and (8) variation of the thermomechanical properties of the ablative material with temperature. Numerical calculations of the surface erosion for H41N have been performed. It is found that particle impact substantially alters the thermochemical ablation and thermal effect also changes the mechanical erosion. The predicted results for overall material erosion of H41N are found to agree reasonably well with available experimental data.

Insulative materials are used as thermal barriers to protect a substrate (e.g., wood, metal, or ordnance) from heat sources (e.g., fires or aerodynamic heating). Insulative coatings can be either inert, ablative, intumescent, or a combination of these three. This paper describes a cost-effective approach that we have developed to screen candidate materials for an application before proceeding with expensive qualification testings. Both radiant heat and hydrocarbon flame environments were used and compared. Several applications are discussed in this paper with emphasis on fast cookoff simulations. Eight insulative materials were tested and compared. Mass loss, maximum expansion, backface temperature, thermal margin, and surface and subsurface analyses were performed. 8 refs.

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.

This study involves the design of a small scale apparatus to test ablative materials, which are commonly used to provide thermal protection to key structural components of solid rocket motors (SRMs). The testing of these materials involves subjecting them to extreme heat and fluid velocities while measuring the resulting temperature on various points of the sample. Observation of the front and back face temperatures during testing, as well as post test analysis of material erosion and mass loss, can provide a quick assessment of the efficacy of newly developed ablative materials. The setup developed through this project is designed to be a small-scale device with quantified heat input and temperature values for the testing of ablatives such as carbon/phenolic composites, C/C composites, conventional polymer composites, and polymer nanocomposites. This device will perform tests on small samples of ablative materials with a front face of roughly 12.7 x 12.7mm (0.5 x 0.5 in.) and 15-50mm thickness. A high temperature flame produced through the use of an oxyacetylene torch system will be directed and focused on this small sample face through the use of a welding nozzle, which will accelerate the flame towards the sample at high velocity. Prior to testing and in order to properly quantify the heat flux generated by this device, a slug calorimeter is used to calibrate the flame by measuring the rate of change of temperature for a slug of material at different distances from the nozzle exit. Data is gathered during testing through the use of multiple thermocouples to measure the back face and/or heat soak temperatures of the sample. These readings are obtained using a data acquisition (DAQ) card and recorded over time using a LabView VI. Due to a specific procedure required to light the oxyacetylene torch, the testing method for this device involves first lighting the torch and adjusting it to the proper fuel ratio, then moving the torch along a single axis slide up to the specified testing distance (used to determine input flux). The data of initial material testing with this apparatus are compared to knowledge of the materials tested, as well as data from tests with a similar apparatus to determine the overall validity of the results gathered from the developed apparatus.