Accepted Manuscript

Title: Multi-functional ULTEMTM 1010 Composite Filaments

for Additive Manufacturing Using Fused Filament Fabrication
(FFF)

Authors: Hao Wu, Michael Sulkis, James Driver, Amado

Saade-Castillo, Adam Thompson, Joseph H. Koo

PII: S2214-8604(18)30576-1
DOI: https://doi.org/10.1016/j.addma.2018.10.014
Reference: ADDMA 537

To appear in:

Received date: 2-8-2018

Revised date: 21-9-2018
Accepted date: 5-10-2018

Please cite this article as: Wu H, Sulkis M, Driver J, Saade-Castillo A, Thompson

A, Koo JH, Multi-functional ULTEMTM 1010 Composite Filaments for Additive
Manufacturing Using Fused Filament Fabrication (FFF), Additive Manufacturing
(2018), https://doi.org/10.1016/j.addma.2018.10.014

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Multi-functional ULTEM™1010 Composite Filaments for
Additive Manufacturing Using Fused Filament Fabrication
(FFF)

Hao Wu, Michael Sulkis, James Driver, Amado Saade-Castillo, Adam Thompson, and Joseph H. Koo

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Department of Mechanical Engineering, The University of Texas at Austin, 204 E Dean Keeton St,
Austin, TX 78712, United States

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*
jkoo@mail.utexas.edu

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Abstract
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

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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-

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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.%
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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-1K-1, an 10.7% improvement in char yield
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and 52% reduction in HRC compared to the neat polymer.

Keywords: Fused Filament Fabrication (FFF), filament, ULTEM, polyetherimide (PEI), flame retardant,
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glass bubbles, nanoclay.

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1. Introduction
As one type of additive manufacturing (AM) method, materials extrusion-based technique such as Fused
Deposition Modeling® (FDM) and Fused Filament Fabrication (FFF) has drawn tremendous interest both
commercially and academically. FFF is a type of free form fabrication process that places molten material
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layer-by-layer to build objects. As illustrated in Figure 1, FFF machines have nozzles that melt and
extrude filament as each layer is added. The platform is usually heated above room temperature to reduce
wrapping and support materials are used to support the weight of a model usually with large spans in the
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horizontal direction [1].

Despite the popular demand, the material selections for FFF printing are still very limited. Traditionally,
materials that can be used for FFF are limited to ABS, PLA, PC, nylon, PEI, and PEEK. The fact that
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majority of the polymers used in FFF are flammable poses a threat when the printed parts are subject to
harsh environment. Low flammability and weight saving are desired properties for many applications in
the aerospace and automotive industries. Therefore, there is a need to develop FFF materials with low
flammability and density. ULTEM™ 1010 is a polyetherimide (PEI) polymer that is flame-retardant and
has high thermal stability.

The study of composite materials for AM in general is widely reported. For example, Weng et al.[3]
studied the mechanical and thermal properties of ABS/MMT clay nanocomposite filament and their 3D

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printed partes. The nanocomposite samples with functionalized MMT nanoclay show significant increase
in both tensile strength and elastic modulus. In addition, the clay nanocomposite samples show higher
degradation temperature and char yield value. Other ABS composite materials including ABS/TiO2[4],
ABS/carbon nanofiber[5] and ABS/graphene[6] have also been reported. It is noticed that many of the
published researches are focusing on modification of traditional AM materials such as ABS and PLA.
Very few studies can be found on material modifications of PEI for extrusion base AM methods. Using
both FDM and injection molding, Chuang et al. [7] did a property comparison of ULTEM™ 9085,
ULTEM™ 1000, and ULTEMTM 1000 with 10% chopped carbon fiber. They found that the length of
original 6 mm carbon fibers was further reduced to 2-3 mm by the extrusion process. Among all the test

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specimens, the 10% AS4 fiber filled ULTEM™ 1000 exhibit the lowest tensile strength and moderate
modulus. This low mechanical strength of the fiber filled composite was due to the high porosity which
was caused by water trapped within the filament as well as degradation at the liquefier in the FDM

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machine. The higher printing temperature for the ULTEM™ 1000 caused by its high melt viscosity
makes it more subject to volume expansion of trapped moisture. Polyakov et al. [8] studied the effect of
adding various loadings (0.5 wt.%, 1 wt.% and 3 wt.%) of carbon nanofibers into ULTEM™ 1000 on the

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mechanical properties of the FDM printed nanocomposite parts. Tensile test results showed that 1 wt.% of
carbon nanofiber improved the tensile strength by 8% as well as modulus by 11% while keeping the
elongation at break at the same level around 9%. Ghose et al. [9] investigated the effect of adding
different nanofillers on thermal conductivity of ULTEM™ 1000 composites. Three types of nanofillers

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including multiwalled carbon nanotubes (MWCNTs), carbon nanofibers (CNF) and expanded graphite
(EG) were used and the concentrations range from 5 to 40 wt%. Effect of nanofiller alignment was also
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studied. Both aligned and unoriented samples show significant increase in thermal conductivity and the
greatest improvement come from the sample with 40wt% of EG when tested along the aligned direction.
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PEI’s characteristics make it an excellent candidate for use in aerospace and automotive parts. One
potential application for such materials is new type Thermal Protection Systems (TPS). The aerospace
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industry relies on the development of new TPS materials as well as new manufacturing techniques of
these materials. TPS materials are vital to the aerospace industry because these materials serve as a heat
shield that protects spacecraft and other vehicles from the intense heat generated during entry into a
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planetary atmosphere. Effective TPS materials combine their high heat flux capacity with low density and
high mechanical strength. However, the production costs of TPS materials by traditional manufacturing
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techniques tend to be prohibitive [10]. In general, the TPS materials can be divided into ablative TPS
materials and non-ablative TPS materials. The ablative TPS can be subcategorized into organic polymeric
and composite materials, and inorganic polymer/oxide and metal materials [11]. One of the largest and
most versatile class of TPS material is polymer-based ablatives [11]. Currently, many organic polymeric
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and composite TPS materials are made of fiber-reinforced polymer matrix composites, and the
manufacturing processes are typically labor intensive and expensive. Therefore, there is a need to develop
advanced TPS materials that can be manufactured quickly and cost effectively. Additive Manufacturing
(AM) provides a novel method of accomplishing this. AM, specifically fused deposition modeling
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(FDM), provides for rapid prototyping and manufacturing of complex parts at a lower cost compared to
traditional manufacturing techniques. Jiang et al. developed PEI nanocomposite foams as TPS materials
[12]. PEI with various concentrations of nanoclay and elastomer were compounded before foamed using a
batch solid-state foaming process. The nanocomposite foams exhibited low thermal conductivities and
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improved thermal stabilities. After ablation, the PEI foams maintained their structural integrity while
porosity and addition of nanoclay help reduced mass loss. This study demonstrated that PEI has the
potential to be used as heat shielding materials for TPS.

To develop a viable material for use in TPS applications, PEI needs to be modified with functional
additives to reduce the density, while still maintaining the level of characteristic thermal and mechanical
properties. To achieve this goal, a total of 3 different functional additives were selected and this study is
intended to explore the possibility of making low density high-performance FDM materials. Glass

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bubbles are hollow microspheres made of chemically stable soda-lime-borosilicate glass. It is an
engineering additive for reducing material density as well as improve melt flow and part warpage. At 110
MPa, the survival ratio of selected glass bubbles is 90% [13]. Its high crush strength allows them to
withstand the melt extrusion process. Moreover, the low thermal conductivity of the glass bubbles will be
beneficial for thermal insulation of the TPS. Numerous studies on polymer nanocomposites have shown
that nanoclay is an effective ingredient for enhancing char yield as well as flame-retardant properties by
forming a protective char layer at the surface [14-16]. Non-halogenated intumescent flame-retardant
provides synergy with nanoclay to improve the char’s insulation and barrier properties [17, 18].

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2. Experimental Approach

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2.1 Materials
ULTEM™ 1010, PEI was purchased from SABIC. Three different additives were used in this study are:
Glass bubbles (GB) iM16K were provided by 3M. The main composition of the glass bubbles is soda

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lime borosilicate glass. The GB is an effective additive for lightweight applications. The calculated
thermal conductivity is 0.153 W/(m-K) at 21℃. Its low thermal conductivity which makes it an ideal
candidate for the TPS application. The D10, D50 and D90 diameter for the particle size distribution are

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12um, 20um and 30um respectively. Nanoclay CLOISITE® Ca++ was provided by BYK Additives and
Instruments. CLOISITE Ca++ is a natural bentonite without modification. The use of non-functionalized

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nanoclay is because majority of the organic functional groups can not withstand the high processing
temperature of PEI polymer. The typical dry particle of the as received nanoclay are less than 10um (D50
diameter). Flame-retardant OP 1312 was provided by Clariant. Exolit® OP1312 is a non-halogenated
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flame-retardant additive based on organic aluminum phosphinates. Its flame-retardant mechanism is
through intumescence, whereby a protective aluminum phosphate barrier is formed to isolate the
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combustion fuel source from heat and oxygen [19]. The decomposition products of melamine phosphate
including ammonia, and CO2 not only dilutes the fuel but also help creates a foamed char structure
working as an effective heat insulating barrier [20]. The PEI polymers was dried at 80℃ overnight to
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eliminate moisture before extrusion, all other materials were used as-received.
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2.2 Formulation Selection

To explore the effect of each additive, we studied four formulations that will provide preliminary data to
give insight about the materials property. The 4 formulations (F) are tabulated in Table 1. Neat PEI (F1) is
used as a control. The main goal for this study is to reduce material density therefore 10 wt.% of glass
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bubbles were added in formulation 2 (F2) which reduces the density close to 1 g/cm3. To enhance the char
formation at extreme heated conditions, 5 wt.% of nanoclay was added in formulation 3 (F3). The weight
loading of nanoclay is based on previous studies. Several groups have confirmed that 5 wt.% loading
could effectively enhance materials char yield thus forming an effective barrier effect during combustion
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[14, 18, 20-23]. Lastly, 10 wt.% of flame retardant additive was added in formulation 4 (F4) to further
improve the char performance. The theoretical density of each formulation was calculated based on the
density of the individual components using the Equation 1:
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1
𝜌𝑎𝑣𝑒 =
𝐶𝑃𝐸𝐼 𝐶𝐺𝐵 𝐶𝑁𝐶 𝐶𝐹𝑅
𝜌𝑃𝐸𝐼 + 𝜌𝐺𝐵 + 𝜌𝑁𝐶 + 𝜌𝐹𝑅 (1)

Where 𝜌𝑎𝑣𝑒 is the calculated average density of the given formulation; 𝐶𝑃𝐸𝐼 , 𝐶𝐺𝐵 , 𝐶𝑁𝐶 , 𝐶𝐹𝑅 are the weight
percentages (wt%) of PEI, glass bubble, nanoclay, and flame retardant respectively; 𝜌𝑃𝐸𝐼 , 𝜌𝐺𝐵 , 𝜌𝑁𝐶 , 𝜌𝐹𝑅
are the densities of each individual component. The densities of each component are: 1.27 g/cm3 for neat

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PEI, 0.46 g/cm3 for the glass bubbles, 2.80 g/cm3 for nanoclay, and 1.45 g/cm3 for flame retardancy. The
calculated densities are shown in Table 1.

2.3 Materials Processing

To manufacture filament of an appropriate size for use in FFF machines. a filament extrusion process
consisting of a twin-screw extruder, water bath, and a customized filament puller was set up as shown in
Figure 2a. To achieve precise speed control, the filament puller consists of a potentiometer, an Arduino
controller and a stepper motor. For extrusion, a Thermo Scientific Process 11 parallel co-rotating twin-

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screw extruder was used. All materials were extruded at 270°C, with a screw speed of 250 rpm. To ensure
a homogenous dispersion, formulations with varying concentrations were stir mixed prior to melt
compounding.

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Before extrusion, the PEI pellets were grinded into powder form to facilitate the feeding process. The
final diameter of the filament was controlled by adjusting the pulling speed. For each formulation, the

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puller speed was adjusted until the filament diameter was approximately 1.75 mm. Initially, the stepper
motor configuration was for standard stepping; however, the required speed was so low that motor
vibrations were affecting the quality of the filament. To mitigate this, the Arduino code was adjusted to
reconfigure the stepper motor into ¼ step micro-stepping mode. This allowed us to operate at low speeds

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with minimal vibrations, which improved the filament consistency. The resultant filaments are shown in
Figures 2b and 2c.

2.4 Characterization
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To better understand the performances of the processed PEI filaments, various characterization methods
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were used in this study. First, the filament quality was characterized by directly measuring filament
diameter. Filament morphology and microstructures were observed using SEM and TEM. Since flame
and thermal performance are of important consideration for this study, TGA an MCC were used to study
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the thermal stabilities and flammability. All characterizations performed in this study are based on the
extruded filaments.
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2.4.1 Filament measurement

Filaments of the four formulations were collected and diameter measurements were taken periodically
throughout the extrusion process. Filament measurement took place at approximately one-minute
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intervals which equals to 0.6m using calipers accurate to 0.01 mm.

2.4.2 Transmission Electron Microscopy (TEM)

TEM was used to examine the dispersion of nanoclay and FR additives. Ultrathin sections of the extruded
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filament samples in the range of 70 nm were obtained by microtoming using a Leica Ultramicrotome with
a diamond knife. The sections were then placed on 400 mesh copper grids. The grids were then put into a
FEI Tecnai Transmission Electron Microscope working at accelerating voltage of 80 kV for imaging.
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2.4.3 Thermogravimetric analysis (TGA)

Thermal decomposition of each polymer blend was assessed by a TGA-50 from Shimadzu Scientific
Instruments. Samples with size between 15-20mg were heated in a nitrogen environment from room
temperature to 1,000°C at a heating rate of 10°C/min. The nitrogen flow was 20 mL/min. Single TGA test
was performed on each formulation and was used to determine the decomposition temperatures at 10%
mass loss (T10%). The char yield is defined as the weight percentage at 1,000°C. PEEK manufactured by
Solvay was also included in the characterization to establish a baseline for comparison.

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2.4.4 Micro-scale Combustion Calorimetry (MCC)
A Micro-scale Combustion Calorimeter (MCC2, Govmark, Inc.) was used to measure the thermal
combustion properties according to ASTM D7309-2007. The combustor temperature was held constant at
900°C and the heating rate of the pyrolysis was 1°C/s. The percentage of oxygen consumption was
measured to calculate the corresponding heat release. All values reported in the discussion are average of
3 tests, stand errors are also reported.

2.4.5 Filament and char morphology analysis using Scanning Electron Microscopy (SEM)
To examine the effect of extrusion on the morphology of the additives, surface and cross section of F2

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containing 10 wt.% glass bubbles was analyzed using SEM. After MCC test, the char residue of all
formulations was collected and analyzed using SEM equipped with Energy Dispersive X-Ray (EDX)
detector to understand the morphology and char formation mechanism. All specimens were analyzed

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using a FEI Quanta 650 ESEM operated at 5kV. To minimize charging, all samples were sputter coated
with gold/palladium prior to SEM analysis.

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3. Results and Discussions

3.1 Filament Measurements

The filament diameter for each formulation is summarized in Table 2. The results show that diameters of

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all filaments are slightly smaller than 1.75 mm, the standard size for most FFF printers. It is worth noting
that the standard deviations for these filaments are relatively low, which means the diameter are uniform

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through its length. Furthermore, the slightly undersized filaments can prevent jamming the heated nozzle
while printing.
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After extrusion, SEM and TEM were used to examine the filament morphology and investigate whether
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the mixtures are uniformly mixed by twin-screw extrusion. Moreover, it is also important to check
whether the glass bubbles remained intact. Figure 3 shows the surface and cross-section of F2 containing
10 wt.% glass bubbles. The filament surface exhibits certain levels of roughness caused by the embedded
glass bubbles. From this image, it is clear that the majority of the glass bubbles did not break during
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extrusion, and that the formulations were uniformly mixed.

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To understand the dispersion of the FR additive and nanoclay, TEM image was also taken on F4
containing all three additives. As shown in Figure 4, all three additives are visible in the TEM image.
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Flame-retardant particles and fragments of the glass bubbles are distributed throughout the sample. The
fragmentation of the glass bubbles was believed to be caused by the microtoming process as part of TEM
sample preparation. Higher magnification image on the right shows that the nanoclay platelets are
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intercalated. This non-exfoliated morphology is expected because of the lack of surfactant on the
nanoclay as discussed in the materials section. Because of the high processing temperature of PEI, non-
surface treated nanoclay was selected on purpose to prevent surfactant degradation.
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Stabilities
The TGA thermal stability studies were conducted on each of the four formulations. Additionally, PEEK
(Solvay KetaSpire® KT-820) was also included in the characterization to establish a baseline for
comparison. Figure 5a (top) shows the TGA mass loss curve of all five specimens. Figure 5b (bottom)
shows the derivative of the TGA data for the five formulations including PEEK. The temperature peaks of
the DTG curve, temperature at 10% mass loss (Tdec 10%), and char yield (wt.% at 1,000°C) results are
listed in Table 3. For TPS materials, one of the important parameters is char yield, higher char yield

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means the materials will be able to maintain its mass at elevated temperatures and therefore protecting the
spacecraft from high heat fluxes.

The Tdec 10% of neat PEI is around 564℃, which is quite high compared to other thermoplastics used in
FFF, such as ABS and PLA [23, 24]. DTG curve of the neat PEI shows a one-step decomposition process
ranging from 550 to 700℃. From Figure 5a, the char yield results indicate that the addition of glass
bubbles does not significantly alter the char yield of PEI but it significantly changed the onset degradation
temperature and peak decomposition temperature by about 40℃ lowers than the control sample. DTG
curve in Figure 5b shows that the peak mass loss rate of F2 was significantly reduced by adding only

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glass bubbles. This indicates that the glass bubbles start to degrade at lower temperature and slower rate
while the char yield is similar to that of neat PEI.

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The inorganic nature gives nanoclay high thermal stabilities [25, 26]. Addition of 5 wt.% of nanoclay in
F3 improved the char yield by about 4% which is close to the amount of nanoclay. It is also noted that
Tdec 10% and peak mass loss temperature were also increased by about 10℃ compared to F2 without

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nanoclay. Lee et al. [27] have demonstrated that PEI nanocomposite with intercalated nanoclay exhibits
higher decomposition temperature than the neat polymer. Adding all three additives as in F4 can improve
the char yield by 6% compared to neat PEI. This is because of the increased amount of inorganic
components, such as silicates from the glass bubbles as well as the nanoclay. The decomposition products

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from the FR also contributes to the char yield. One major change in thermal stability for F4 is the
decreased onset decomposition temperature. The lower onset of decomposition temperature of F4 is
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attributed to the addition of the intumescent flame retardant in this formulation [14]. It is worth noting
that the char yield for all PEI formulations including the neat control are higher than that of neat PEEK. In
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general, the TGA results indicate that the addition of the additives improves the char yield and decreased
the mass loss rate as well as decomposition temperature.
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3.3 Flammability Results Characterized by MCC

Micro-scale Combustion Calorimetry or MCC, developed by the FAA, is a robust flammability
characterization tool because of its small sample size requirement. Heat Release Capacity 𝜂𝑐 as defined in
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Equation 2 can be calculated from MCC results. The HRC is an intrinsic material property reflecting the
material’s maximum potential to release heat assuming complete combustion [21].
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𝑄𝑚𝑎𝑥
𝜂𝑐 = (2)
𝛽
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Where 𝑄𝑚𝑎𝑥 is the peak heat release rate W/g, and 𝛽 is the heating rate K/s.

Figure 6 shows the representative heat release curves of all formulations including PEEK. Neat PEI as a
benchmark have an HRC value of 247 J/g-1K-1 which is already significantly lower than ABS (581 J/g-1K-
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), PA66 (623 J/g-1K-1) [28], and even PEEK (359 J/g-1K-1). A comparison of the HRC values of all
specimens are shown in Figure 7. It is noted that the addition of glass bubbles significantly reduced the
HRC to 170 J/g-1K-1. One possible mechanism for this reduction is that these glass hollow spheres broke
at elevated temperatures during combustion and this endothermic process inhibits the heat release. The
barrier effect of the broken glass bubbles could also help further reduce the heat release by forming a
reinforced protective char layer, blocking the fuel path as well as heat conduction. It is somewhat
surprising to see the addition of 5 wt.% nanoclay results in a slight increase in the HRC to 189 J/g-1K-1.

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Although the reason for this is not fully understood yet, it is reasonable to attribute this phenomenon to
the poor dispersion of the nanoclay as will be discussed in the following section.

Finally, combining all three additives including glass bubbles, nanoclay, and FR, F4 achieved significant
reduction in HRC to 119 J/g-1K-1, a 52% decrease when compared to the neat control. This large reduction
in HRC is due to the flame retardant is expected as the synergistic effect between nanoclay and non-
halogenated intumescent FR is well documented [29]. The reduction in heat release rate corresponds to a
reduction in the flammability of the formulation. The key values from the MCC are summarized in Table
4. Due to the decreasing amount of organic content, the total heat release decreases as more additives

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were added in the formulation. Incorporation of glass bubbles lowered the peak HRR temperature by 30℃
but as the nanoclay and FR were added consequently, the peak HRR temperature was brought back to
560℃. Overall, the MCC data show that F4 is the best performing formulation with regards to

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flammability.

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3.4 Microstructural Analysis of Char Residue

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An important feature of ablative TPS materials is the formation of a protective char layer. The char layer
provides insulating protection and inhibits releasing of the pyrolysis gases, which protects the
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components underneath [30]. After MCC testing, each of the pyrolyzed PEI char residue was analyzed
using SEM and EDX to better understand the physical characteristics of the char layer. Figure 8 shows the
char surface of F1-F4, respectively. Neat PEI formed a quite uniform and nearly featureless char layer.
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This void free solid char morphology is an indication of the high thermal oxidative stability of PEI and is
beneficial for the ablation application. Figure 8b shows the important features in the char of F2. Similar to
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the neat PEI, the char surface is composed of a full covered char layer with high structural integrity. The
char layer is reinforced by a mixture of intact and broken glass bubbles.
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A zoomed in image shows the embedder glass bubbles in the char layer. The presence of intact and
broken glass bubbles aid in the formation of a more robust char layer which lead to lower heat release
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rate. This assumption is proved by the MCC results discussed in previous section. The char layer of F3
shown in Figure 8c is quite identical F2. The only difference is the presence of a small number of
nanoclay agglomerates highlighted in the image. The intercalated nanoclay is not visible at this
magnification. Due to the intumescent FR additives, the char layer surface roughness of F4 is relatively
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high compared to the other formulations (Figure 8d). Glass bubbles as well as a high concentration of FR
additive is visible from the surface.
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Shown in Figure 9, representative SEM images were taken on the char layer cross section of F3 and F4.
Both char layers exhibit a solid structure through the thickness. While intact and broken glass bubbles
coexist within the char layer, it seems the majority of the glass bubbles broke during combustion.
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To better understand the char morphology, EDX mapping of a representative area on the char surface was
carried out of F4 shown in Figure 10. The EDX spectra show distinct peaks of carbon, oxygen,
phosphorus, zinc, aluminum, silicon, and calcium. Among these elements, carbon and oxygen are the
main building blocks of the carbonaceous char residue; silicon and oxygen are the major components of
glass bubbles; phosphorus, zinc, aluminum comes from the FR additive; and aluminum, silicon. and
calcium are contained in the nanoclay.

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Based on the above assumption, silicon and aluminum mapping results confirmed that the large particle
(indicated by the orange arrow on the upper left corner) embedded in the char layer was an agglomerate
of nanoclay because it is the only component that contains both aluminum and silicon. Silicon mapping of
the surrounding smaller particles confirmed that these are indeed broken glass bubbles. Finally, mapping
of the phosphorus indicates that the small particle at the bottom left side of the large nanoclay cluster
comes from FR additive.

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4. Conclusions

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In this study, a series of multi-functional composite FFF filaments based on PEI was developed for
additive manufacturing of ablative Thermal Protection System (TPS). A total of four formulations
including the neat control were processed by twin-screw extrusion. Filaments with diameter close to 1.75

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mm were collected and characterized. It was observed from SEM and TEM that most of glass bubbles
remained intact after the extrusion process and the nanoclay platelets are not exfoliated due to lack of
surface functionalization. In general, the filaments had uniform additive dispersion to produce consistent
material properties throughout the length of the filament.

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TGA data showed that the addition of nanoclay and flame retardant resulted in a higher char yield at a
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lower mass loss rate. The highest char yield came from the formulation that contained glass bubbles,
nanoclay, and flame-retardant additives. Addition of any additives will reduce the onset decomposition
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temperature. The MCC characterization data showed that the addition of glass bubbles, nanoclay, and
flame retardant are successful in producing a lower flammability material. A 52% reduction in HRC was
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achieved from the same formulation with the highest char yield. Analyzing the differences between the
post-extrusion and post-char imaging shows that majority of the microbubbles broke during combustion.
The broken glass bubbles act as reinforcement to the already solid char layer, resulting in even lower
flammability.
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The results of this study show a great potential for the PEI composite material to be used in additive
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manufacturing of TPS via extrusion-based AM method. The most promising formulation consisted of 75
wt.% PEI, 10 wt.% glass bubbles, 5 wt.% nanoclay, and 10 wt.% flame retardants. To better understand
the materials performance for this developed PEI composite materials, the results presented in this paper
will guide future optimization and characterizations on the FFF printed parts.
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Acknowledgements
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The authors are grateful to KAI, LLC for sponsoring this project. The authors would also like to thank
Clariant, 3M, and BYK for helpful discussions as well as providing the materials for this study.
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References

[1] M.J. Richardson, H. Wu, T.J. Wilcox, M. Broaddus, P.C. Lin, M. Krifa, J.H. Koo, Flame Retardant
Nylon 6 Nanocomposites for Fused Deposition Modelling (FDM) Applications, SAMPE 2017, Seattle,
2017.
[2] D.S. Ertay, A. Yuen, Y. Altintas, Synchronized material deposition rate control with path velocity on
fused filament fabrication machines, Additive Manufacturing 19 (2018) 205-213.
[3] Z. Weng, J. Wang, T. Senthil, L. Wu, Mechanical and thermal properties of ABS/montmorillonite
nanocomposites for fused deposition modeling 3D printing, Materials & Design 102 (2016) 276-283.

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[4] A.R.T. Perez, D.A. Roberson, R.B. Wicker, Fracture surface analysis of 3D-printed tensile specimens
of novel ABS-based materials, Journal of Failure Analysis and Prevention 14(3) (2014) 343-353.
[5] M.L. Shofner, K. Lozano, F.J. Rodríguez-Macías, E.V. Barrera, Nanofiber-reinforced polymers

RI
prepared by fused deposition modeling, Journal of Applied Polymer Science 89(11) (2003) 3081-3090.
[6] S. Dul, L. Fambri, A. Pegoretti, Fused deposition modelling with ABS–graphene nanocomposites,
Composites Part A: Applied Science and Manufacturing 85 (2016) 181-191.

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[7] K.C. Chuang, J.E. Grady, R.D. Draper, E.-S.E. Shin, C. Patterson, T.D. Santelle, Additive
Manufacturing and Characterization of Ultem Polymers and Composites, CAMX Conference
Proceedings. Dallas, TX, October 26-29, 2015. (2015).

U
[8] I.V. Polyakov, G.V. Vaganov, V.E. Yudin, E.M. Ivan’kova, E.N. Popova, V.Y. Elokhovskii,
Investigation of Properties of Nanocomposite Polyimide Samples Obtained by Fused Deposition
N
Modeling, Mechanics of Composite Materials 54(1) (2018) 33-40.
[9] S. Ghose, K.A. Watson, D.M. Delozier, D.C. Working, J.W. Connell, J.G. Smith, Y.P. Sun, Y. Lin,
Thermal conductivity of Ultem™/carbon nanofiller blends, American Society of Mechanical Engineers,
A
pp. 237-245.
M

[10] D. Glass, Ceramic matrix composite (CMC) thermal protection systems (TPS) and hot structures for
hypersonic vehicles, 15th AIAA International Space Planes and Hypersonic Systems and Technologies
Conferencec, 2008, p. 2682.
D

[11] M. Natali, J.M. Kenny, L. Torre, Science and technology of polymeric ablative materials for thermal
protection systems and propulsion devices: A review, Progress in Materials Science 84 (2016) 192-275.
TE

[12] W. Jiang, S.S. Sundarram, D. Wong, J.H. Koo, W. Li, Polyetherimide nanocomposite foams as an
ablative for thermal protection applications, Composites Part B: Engineering 58 (2014) 559-565.
[13] 3M, 3M™ Glass Bubbles K Series, S Series and iM Series, 2018.
[14] H. Wu, R. Ortiz, A. Correa Renan De, M. Krifa, J. H. Koo, Self-Extinguishing and Non-Drip Flame
EP

Retardant Polyamide 6 Nanocomposite: Mechanical, Thermal, and Combustion Behavior, Flame

Retardancy and Thermal Stability of Materials, 2018, p. 1.
[15] H. Wu, M. Krifa, J.H. Koo, Flame retardant polyamide 6/nanoclay/intumescent nanocomposite fibers
CC

through electrospinning, Textile Research Journal (2014).

[16] F. Samyn, S. Bourbigot, C. Jama, S. Bellayer, S. Nazare, R. Hull, A. Fina, A. Castrovinci, G.
Camino, Characterisation of the dispersion in polymer flame retarded nanocomposites, European Polymer
Journal 44(6) (2008) 1631-1641.
A

[17] S. Bourbigot, S. Duquesne, G. Fontaine, S. Bellayer, T. Turf, F. Samyn, Characterization and

reaction to fire of polymer nanocomposites with and without conventional flame retardants, Molecular
crystals and liquid crystals 486(1) (2008) 325-1367.
[18] S. Bourbigot, S. Duquesne, C. Jama, Polymer Nanocomposites: How to Reach Low Flammability?,
Macromolecular Symposia 233(1) (2006) 180-190.
[19] U. Braun, B. Schartel, M.A. Fichera, C. Jäger, Flame retardancy mechanisms of aluminium
phosphinate in combination with melamine polyphosphate and zinc borate in glass-fibre reinforced
polyamide 6,6, Polymer Degradation and Stability 92(8) (2007) 1528-1545.

9
[20] H. Wu, R. Ortiz, H. Koo Joseph, Rubber toughened flame retardant (FR) polyamide 11
nanocomposites Part 1: the effect of SEBS-g-MA elastomer and nanoclay, Flame Retardancy and
Thermal Stability of Materials, 2018, p. 25.
[21] J.W. Gilman, T. Kashiwagi, J.D. Lichtenhan, Nanocomposites: a revolutionary new flame retardant
approach, Sampe Journal 33 (1997) 40-46.
[22] F. Laoutid, L. Bonnaud, M. Alexandre, J.M. Lopez-Cuesta, P. Dubois, New prospects in flame
retardant polymer materials: From fundamentals to nanocomposites, Materials Science and Engineering:
R: Reports 63(3) (2009) 100-125.
[23] H. Ma, L. Tong, Z. Xu, Z. Fang, Y. Jin, F. Lu, A novel intumescent flame retardant: Synthesis and

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application in ABS copolymer, Polymer Degradation and Stability 92(4) (2007) 720-726.
[24] F.D. Kopinke, M. Remmler, K. Mackenzie, M. Möder, O. Wachsen, Thermal decomposition of
biodegradable polyesters—II. Poly(lactic acid), Polymer Degradation and Stability 53(3) (1996) 329-342.

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[25] J.H. Koo, Fundamentals, Properties, and Applications of Polymer Nanocomposites, Cambridge
University Press, Cambridge, UK, 2016.
[26] J.W. Gilman, Flammability and thermal stability studies of polymer layered-silicate (clay)

SC
nanocomposites, Applied Clay Science 15(1-2) (1999) 31-49.
[27] J. Lee, T. Takekoshi, E.P. Giannelis, Fire retardant polyetherimide nanocomposites, Materials
Research Society, 1997, pp. 513-518.
[28] R.E. Lyon, R.N. Walters, S.I. Stoliarov, Screening flame retardants for plastics using microscale

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combustion calorimetry, Polymer Engineering and Science 47(10) (2007) 1501-1510.
[29] S. Bourbigot, S. Duquesne, Fire retardant polymers: recent developments and opportunities, Journal
of Materials Chemistry 17(22) (2007) 2283-2300.
N
[30] G.C. April, R.W. Pike, E.G. Del Valle, On chemical reactions in the char zone during ablation,
A
NASA Langley Research Center, 1971.
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Figure caption

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Figure 1. Illustration of FFF setup [2].
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Figure 2. (a) Filament extrusion setup, (b) picture of neat PEI filament, and (c) picture of filaments
of all 4 formulations.
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 Figure 3. SEM image of surface and cross section of filament containing 10 wt.% glass bubbles.

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Figure 4. TEM of F4 with 10 wt.% glass microbubbles, 5 wt.% nanoclay, and 10 wt.% flame
retardant.

3.2 Thermal
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Figure 5. (a) TGA (top) and (b) DrTGA (bottom) results for the four PEI formulations and PEEK.
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Figure 6. Representative MCC Heat release rate curves for the four PEI formulations and PEEK.
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Figure 8. SEM images of post MCC char surface of the four PEI formulations (a) neat PEI, (b) F2,
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(c) F3, and (d) F4.

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Figure 9. Cross section SEM of char layer of F3 (left) and F4 (right).

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Figure 10. Char residue EDX mapping results for F4.

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Table

Table 1. Formulation matrix and its calculated density

Flame Theoretical
Glass Bubbles Nanoclay
Formulation PEI (wt.%) Retardant Density
(wt.%) (wt.%)
(wt.%) (g/cm3)
F1 100 0 0 0 1.27
F2 90 10 0 0 1.08
F3 85 10 5 0 1.09

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F4 75 10 5 10 1.11

Table 2. Summary of filament diameter of all four formulations

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Formulation Average Diameter Standard Deviation (mm)
(mm)

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F1. Neat PEI 1.62 0.07

F2. 10% Glass bubbles 1.68 0.07

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F3. 10% Glass bubbles-5% 1.66 0.07
Nanoclay

F4. 10% Glass bubbles-5%

Nanoclay-10% Flame
1.70 N 0.06
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Table 3. Summary of TGA results

Formulation Tdec 10% (℃) Peak Mass Loss Char Yield
Temperature (wt.%)
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(℃)
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F1. Neat PEI 564 570 56.2

F2. 10% Glass bubbles 522 532 56.3
F3. 10% Glass bubbles-5% Nanoclay 536 544 59.1
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F4. 10% Glass bubbles-5% Nanoclay-

10% Flame Retardant 535 549 62.2
F5. PEEK 610 618 52.1
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Table 4. Summary of MCC results

Formulation Heat Release Peak
Peak HRR Total HR
Capacity Temperature
(W/g) (W/g)
(J/g-1K-1) (℃)
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F1. Neat PEI 248±3 247±4 7.3±0.2 571±1

F2. 10% Glass bubbles 170±1 170±0 6.9±0.1 534±0
F3. 10% Glass bubbles-5%
189±1 189±1 6.3±0.2 551±0
Nanoclay
F4. 10% Glass bubbles-5%
Nanoclay-10% Flame 119±1 119±1 5.1±0.1 560±1
Retardant

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F5. Neat PEEK 359±4 358±4 7.9±0.1 626±0

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