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Ablative materials are used as thermal protection systems (TPS) for reentry vehicles and solid rocket motor (SRM) nozzle applications. Phenolic and cyanate ester are the state-of-the-art (SOTA) resin systems used in many of the ablative composites today, including MX-2600 (silica/phenolic) from Cytec Solvay Group. While these ablatives have worked well, more demanding requirements drive the need for affordable lightweight advanced composites capable of handling high heat fluxes with minimal mass loss. These advanced ablative composites result in lighter reentry heat shields and solid rocket motors, increasing payload capabilities of spacecraft and rockets. Molding compound made of aerospace grade 99% SiO2 fabric and polysiloxane resin showed considerable improvement over MX-2600 in ablation properties in recent studies. In order to meet increased mechanical strength demands, NASA recently developed an ablative composite using a 3D quartz woven/cyanate ester composite material designed for the Orion spacecraft. While 3D woven composites provide excellent out-of-plane mechanical and ablation properties, they are very expensive, which limits their application. This research explores needle-punched silica fabric, sometimes referred to as 2.5D, which provides similar out-of-plane mechanical benefits to 3D woven composites in a more flexible VARTM manufacturing process at a much lower cost. The needle-punched silica fabric was infiltrated with polysiloxane resin and mechanical tests were performed. The needle-punched composites showed an increase of 181% in flexural strength, 27% in interlaminar shear strength, 2% in tensile strength, and 13% in compressive strength. In aerothermal ablation tests, the 2.5D out-performed the 2D laminate in char yield, mass loss, and recession rate; and in char yield and mass loss (%), the 2.5D out-performed the industry standard MX-2600 molding compound. The increased out-of-plane strength and char yield make it a promising and affordable ablative candidate for ablation performance with enhanced mechanical properties.

A novel class of ablative materials using phenolic resin, silica fabric, and nano-zirconium dioxide is being developed as a potential alternative for use in high-heat-flux thermal protection applic...

As additive manufacturing (AM) increasingly gains commercial and academic interest, government agencies, such as NASA Johnson Space Center, seek to produce thermal protection systems using AM methods, such as fused filament fabrication (FFF). The purpose of this study is to develop a suitable polyetherimide (ULTEM™ 1010) nanocomposite with enhanced ablation and thermal properties while maintaining compatibility with commercially available FFF machines. Eleven formulations were compounded using the twin-screw extruder with varying amounts of nanoclay, glass bubbles, and flame-retardant additives. All formulations were characterized via multiple instruments to be down-selected to four candidates for aerothermal testing. After discovering specific characteristics of some formulations, aerothermal test models were 3D printed for four formulations. Enhanced thermal properties among the composite formulations were demonstrated. Aerothermal testing also displayed promising results. In addition to experimental data, microstructural analyses were performed.

In this study, mathematical formulations to model charring ablation problems were numerically implemented using finite element analysis (FEA) with ABAQUS, which account for the material decomposition and progressive surface removal in the heat conduction and the surface energy balance equations. FEA was performed for a one-dimensional model to predict the temperature and ablation histories of a phenolic-impregnated carbon ablator sample (i.e., a common heat shield material for hypersonic vehicles and spacecraft) subjected to oxy-acetylene torch flame (i.e., 0.8 SLPM acetylene gas to 2.7 SLPM oxygen gas). The recovery enthalpy and convective heat transfer coefficient for the ablation model were calculated based on gas compositions and two assumed surface conditions (i.e., equilibrium and frozen). Simulations using the calculated recovery enthalpy and convective heat transfer coefficient resulted in a recession rate of 6.38 times (equilibrium) and 14.08 times (frozen) higher than the experimental data, despite fair agreement of the surface temperature. In addition, the effect of the heat transfer coefficient was investigated through a steady-state ablation analysis. The results of the analysis indicate that there is not one single value for the heat transfer coefficient that would allow the prediction to match both measured recession rate and surface temperature. Possible reasons for such an inconsistency are provided and discussed.

Thick-section composites (TSC) are extensively demanded in many fields, such as aerospace, wind energy, and oil and gas industries. However, the manufacturing process of thick-section thermoset composites (TSSC) encounters significant complexities, such as variations of nonuniform resin flow, exothermal reaction and curing, and dimensional stability through the thickness direction. These process-related nonuniformities are expected to result in through-thickness gradients of mechanical properties and curing-induced deformations, leading to undesirable residual stresses and damage. This chapter introduces the application of TSC and issues related to its manufacturing processes. Methods of TSC are examined and analyzed. Fundamental characteristics of curing kinetics, thermal transfer, and residual stress in TSC will be explained. Research of detailed experiments will be referred for readers for further studies.

In current thermal protective clothing systems, maximizing the personal protection performance against fire and heat is in great demand. Additionally, minimizing the manufacturing cost by using low-cost materials is also a critical factor. In previous studies [1–3], a new type of low-cost inherently flame resistant (FR) non-drip Polyamide 6 (PA6) nanocomposite fiber was developed. In this paper, this FR non-drip PA6 fiber was tested in blends with two commercially available inherently FR fibers to form FR nonwoven fabrics. Different formulations of varying blending ratios were processed into nonwoven fabrics. The fiber morphology was observed by Scanning Electron Microscopy (SEM). The fabric flammability and combustion properties were characterized using a Microscale Combustion Calorimeter (MCC), and a vertical flame tester, as well as Thermogravimetric Analysis (TGA). Tensile tests were conducted to characterize mechanical properties of these FR nonwoven fabrics. The water vapor permeability test was also performed to measure the wearability of the fabric. Results of several nonwoven blends were compared to find the one with optimum blend ratio which has the potential to be used as low-cost thermal protective fabric.

This paper investigates the development of a novel high temperature polymer composite material by modifying polyetherimide (PEI) ULTEM™ 1010 with the addition of functional additives and processing it into filaments for Fused Filament Fabrication (FFF). Through twin-screw extrusion, four different formulations were obtained using combinations of hollow glass microspheres, nanoclay, and non-halogenated flame-retardant additives. These additives were designed to create a material that exhibits low density, high char yield, and low flammability. Filament quality was characterized and reported. Thermal and flammability characterization results indicated that the formulation consisting of 10 wt.% glass bubbles, 5 wt.% nanoclay, and 10 wt.% flame-retardant additives exhibited the best char yield at 62.2% and the lowest heat release capacity (HRC) of 119 J/g−1 K−1, an 10.7% improvement in char yield and 52% reduction in HRC compared to the neat polymer.

This paper provides the details of the R&D of a new ablative material through a collaboration between The University of Texas at Austin and Techneglas LLC. The UHTR polysiloxane resin system is man...

The objective of this research is to develop a multifunctional polyamide 11 (PA11) with balanced thermal, mechanical, and flammability properties for SLS. In this study, two sets of formulations were prepared by twin-screw extrusion: the first set examined the effect of maleic anhydride modified elastomers on flammability and the mechanical properties, whereas the second set added various amount of nanoclay and discussed thermal stability, flammability and mechanical properties. The addition of 20 wt.% elastomer brought the elongation at break up to 40%. Reduction in heat release capacity as high as 49% was achieved, all nanocomposite samples passed UL 94 V-1 rating. The addition of nanoclay improved the tensile modulus by up to 78%, the elongation at break for all the formulations were negatively affected by the addition of flame retardant and nanoclay.

In this study, inherently flame retardant nanocomposite nylon 6 fibers infused with nanoclay and intumescent additives were compounded and melt-spun. Two approaches were adopted to mitigate the loss of mechanical properties typically observed nanocomposite fiber systems: (a) additive particle size reduction; and (b) elastomer toughening of the nanocomposite system. As a result, the ductility of the FR nanocomposite formulations was improved significantly. Structural and morphological characterization of the melt-spun fibers using TEM and XRD demonstrated good dispersion of the additives and exfoliation of the nanoclay platelets. Microscale Combustion Calorimetry analysis demonstrated effective reduction of heat release capacity and thus significant enhancement of flame retardant performance of the compounded fibers.

In this research, a break-wire-like ablation recession and thermal sensor based on commercial, affordable thermocouples was designed, fabricated, and tested. The objective of this paper was to demo...

Incorporation of flame-retardant (FR) additives and nanoclay fillers into thermoplastic polymers effectively suppresses materials flammability and melt dripping behavior. However, it largely affects other properties, such as toughness and ductility. In order to recover the lost toughness and ductility of flame retardant polyamide 6, various loadings of maleic anhydride modified SEBS elastomer were added and processed by twin screw extrusion. TEM images showed exfoliated nanoclay platelets and reveals that the clay platelets well dispersed in the polymer matrix. By balancing the ratio of flame retardants, nanoclay and elastomers, formulation with elongation at break as high as 76% was achieved. Combining conventional intumescent FR and nanoclay, UL-94 V-0 rating and the LOI value as high as 32.2 were achieved. In conclusion, effective self-extinguishing and non-drip polyamide 6 nanocomposite formulations with significant improvement in toughness and ductility were achieved.

In this study, elastomer toughening is explored as a means of recovering ductility of polyamide 6/intumescent flame-retardant composite systems. This was achieved using a maleic anhydride modified ...

In order to determine ablative properties, various labscale tests can be performed using different test apparatus. One of the more common test apparatus is the oxy-acetylene torch. In this study, the flow field of an oxy-acetylene test setup is examined using Ansys/Fluent 19.1 code. The heat transfer between the ablative surface and the combustion gases are compared with experimental results. Moreover, the results obtained from the CFD by applying different kinds of turbulence models are discussed. The calculated temperature and heat flux results are in reasonable agreement with the experimental results.

Polymer processing techniques have experienced continued progress over decades to meet performance requirements in diverse applications. In the same period, various high-performance polymeric materials have been developed rapidly. Fabrication of polymer fibers has been in the center of attention from the origin of synthetic polymers, with applications ranging from textiles to biomedical materials. Electrospinning is a well-known process for developing continuous fibers with diameters of tens of microns to hundreds of nanometers. It benefits from special privilege of in-situ modification/functionalization of polymers and nanoscale materials. Flame retardancy is an important performance characteristic of polymer materials. Both material and fabrication aspects have to be taken into account when designing flame retardant polymer materials, but the latter has been rarely dealt with by researchers. This review attempts to categorize state-of-the-art features of flame-retardant polymer materials developed by using electrospinning. Electrospun fibers of natural and synthetic polymers used in developing flame-retardant materials were reviewed and their mechanical properties, advantages and challenging aspects were discussed.

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The study is focused on carbon nanostructure (CNS), including both carbon nanotubes (CNTs) and graphene nanoplatelets (GNP), reinforcement of medical‐grade polyetheretherketone (PEEK) and in‐vitro bioactivity for biomedical structural applications. CNS/PEEK scaffolds and bulk specimens, realized via fused filament fabrication (FFF) additive manufacturing are assessed primarily in the low‐strain linear‐elastic mechanical regime. 3D printed PEEK nanocomposites are found to have enhanced mechanical properties in all cases while maintaining the desired degree of crystallinity in the range of 30‐33%. A synergetic effect of the CNS and sulfonation towards bioactivity is observed– apatite growth in simulated body fluid increased by 57 and 77%, for CNT and GNP reinforcement, respectively, doubling the effect of sulfonation and exhibiting a fully‐grown mushroom‐like apatite morphology. Further, CNT‐ and GNP‐reinforced sulfonated PEEK recovers much of the mechanical losses in modulus and strength due to sulfonation, in one case (GNP reinforcement) increasing the yield and ultimate strengths beyond the (non‐sulfonated) printed PEEK. Additive manufacturing of PEEK with CNS reinforcement demonstrated here opens up many design opportunities for structural and biomedical applications, including personalized bioactivated surfaces for bone scaffolds, with further potential arising from the electrically‐conductive nanoengineered PEEK material towards smart and multifunctional structures.

In recent years, there have been significant advances on materials development for additive manufacturing (AM) applications. However, the use of composites or nanocomposite materials for improved performance and multifunctionality are still limited. This review paper attempts to provide a comprehensive review of both commercially available materials as well as research activities related to recent progress on high-performance polymer nanocomposites that are being used in various AM techniques. Four AM techniques including Fused Filament Fabrication (FFF), Selective Laser Sintering (SLS), Multi Jet Fusion (MJF), and Stereolithography (SLA) are discussed. The development of printable polymer composites especially polymer nanocomposites is rapidly expanding the AM materials portfolio, which makes the production of multifunctional parts with complex structures possible.

This chapter deals with the use of thermal protection materials for two key components of a solid rocket motor (SRM): the exhaust nozzle and internal motor insulation. The role of the materials utilized for SRM thermal protection depends on the engineers involved, who would ideally be using data obtained from full-scale ground or in-flight testing to help guide their choices. Three important research areas are discussed in this chapter: (1) advanced polymer nanocomposite (PNC) ablatives: PNCs for SRM nozzles and PNCs for SRM internal insulation, (2) new sensing technology: in situ ablation recession and thermal sensor, and char strength sensor, and (3) technologies needed to advance PNC ablative research: thermophysical properties characterization and ablation modeling.

© 2019 The Authors The study is focused on multifunctional performance of carbon nanotubes (CNT) and Graphene nanoplatelets (GNP) reinforced PEEK composites enabled via fused filament fabrication (FFF) additive manufacturing (AM) utilizing in-house nanoengineered filaments. Thermo-physical, mechanical and wear characteristics of electro-conductive PEEK nanocomposites are reported. The coefficient of thermal expansion (CTE) is found to decrease by 26% and 18% with the incorporation of 5 wt% GNP and 3 wt% CNT into PEEK polymer, respectively. The decrease in CTE provides better dimensional stability to resulting nanocomposite structures. Due to uniform dispersion of CNT and GNP in the PEEK matrix, the crystallization temperature and degree of crystallinity are both increased. The 3D printed PEEK nanocomposites reveal interfacial voids between the beads and intra-bead pores and thus exhibit lower density compared to that of the 3D printed neat PEEK. Young's and storage moduli are found to increase by 20% and 66% for 3 wt% CNT loading and by 23% and 72% for 5 wt% GNP loading respectively. However, the PEEK nanocomposites exhibit similar tensile strength to that of neat PEEK. The coefficient of friction obtained from fretting wear tests is found to decrease by 67% and 56% for 1 wt% CNT and 3 wt% GNP loaded PEEK nanocomposites, respectively and the decrease is attributed to reduced hardness and increased porosity. Multifunctional performance of carbon nanostructures reinforced AM-enabled PEEK composites demonstrated here makes them suitable for a range of applications such as orthopedics, oil and gas, automotive, electronics and space.

High electrical and thermal conductivity associated with high stiffness and strength offer tremendous opportunities to the development of a series of carbon nanotube incorporated composite materials for a variety of applications. In particular, a small amount of carbon fibers or carbon nanotubes in a non-conductive polymer will transform a composite into a conductive material, which reveals superb potential of their future application in electronic devices. The relation between the amount of carbon nanotubes in a polymer and the electrical conductivity of it can be studied experimentally as well as theoretically with various simulation models. A three-dimensional (3D) Monte Carlo simulation model using resistance network formation was developed to study the relation between the electrical conductivity of the polymer nanocomposite and the amount of carbon nanotubes dispersed in it. In this model, carbon nanotubes were modeled as curvy cylindrical nanotubes with various lengths and fixed tube diameter, all of which were randomly distributed in a non-conductive constrained volume, which represents polymer. The model can be used to find the volumetric electrical resistance of a constrained cubic structure by forming a comprehensive resistance network among all of the nanotubes in contact. As more and more nanotubes were added into the volume, the electrical conductivity of the volume increases exponentially. However, once the amount of carbon nanotubes reached about 0.1 % vt (volume percentage), electrical percolation was detected, which was consistent with the experimental results. This model can be used to estimate the electrical conductivity of the composite matrix as well as to acquire the electrical percolation threshold.

To improve the flame retardancy and mechanical properties of epoxy (EP), a novel intercalated layered double hydroxides (ILDHs) were synthesized by a solution intercalation method using ammonium alcohol polyvinyl phosphate as intercalator. The ILDHs were examined by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), energy-dispersive spectrometry and scanning electron microscopy (SEM). The conditions for synthesis of ILDHs have been optimized. Thermogravimetric analysis indicated a good char-forming property of ILDHs. The flame retardancy of epoxy was improved by adding ILDHs. The EP composites containing 30–40 mass% ILDHs passed UL-94V-0 rating. Tensile strength was also enhanced by adding 30–40 mass% ILDHs, which was attributed to the involvement of physical cross-linking network among layered particles and polymer chains. The morphology and structures of residues generated during LOI test were investigated by SEM and FTIR to support a fundamental analysis for the mechanism of char formation. SEM observations of residues of the ILDHs/EP confirmed the formation of an incompact charred layer during combustion, which could inhibit the transmission of heat and mass during combustion. It may be inferred that ILDHs acted as a catalyst for esterification, dehydration and compact char formation of EP system.