Lee W. Kohlman

Glenn Research Center, Cleveland, Ohio

Charles Bakis

Pennsylvania State University, University Park, Pennsylvania

Tiffany S. Williams, James C. Johnston, Maria A. Kuczmarski, and Gary D. Roberts

Glenn Research Center, Cleveland, Ohio

Engineered Polymer Composites Through

Electrospun Nanofi ber Coating of Fiber Tows

NASA/TM—2014-216635

February 2014
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Lee W. Kohlman

Glenn Research Center, Cleveland, Ohio

Charles Bakis

Pennsylvania State University, University Park, Pennsylvania

Tiffany S. Williams, James C. Johnston, Maria A. Kuczmarski, and Gary D. Roberts

Glenn Research Center, Cleveland, Ohio

Engineered Polymer Composites Through

Electrospun Nanofi ber Coating of Fiber Tows

NASA/TM—2014-216635

February 2014

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2014-216635 1 

Engineered Polymer Composites Through Electrospun 

Nanofiber Coating of Fiber Tows 

Lee W. Kohlman 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Charles Bakis 

Pennsylvania State University 

University Park, Pennsylvania 16802 

Tiffany S. Williams, James C. Johnston, Maria A. Kuczmarski, and Gary D. Roberts 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Composite materials offer significant weight savings in many aerospace applications. The toughness 

of the interface of fibers crossing at different angles often determines failure of composite components. A 

method for toughening the interface in fabric and filament wound components using directly electrospun 

thermoplastic nanofiber on carbon fiber tow is presented. The method was first demonstrated with limited 

trials, and then was scaled up to a continuous lab scale process. Filament wound tubes were fabricated 

and tested using unmodified baseline towpreg material and nanofiber coated towpreg. 

Introduction 

Composite materials are typically constructed of a fiber and matrix, with the fiber placed at various 

angles and positions within the matrix. In a unidirectional laminate, the strength of the material is 

primarily determined by the strength of the fiber, strength of the matrix, adhesion of the fiber to the 

matrix, fiber volume fraction, and the presence of defects. Since the fiber direction strength relies on load 

carried by the fiber, the lamina is much stronger in the fiber direction than in any direction orthogonal to 

the fibers. Multi-directional laminates use many unidirectional lamina layers at different orientations to 

combine the high fiber direction strength with the low transverse strength to achieve suitable bulk 

properties. These materials rely on the transfer of shear loads across an interface between the lamina. 

Ultimately, when this interface fails, the composite will likely fail. Features such as edges, holes, or 

delamination due to damage often serve as the initiation point for the failure of the interface. Components 

with more complex architectures such as those produced with fabrics including weaves or braids, or by 

filament winding limit the propagation of catastrophic delamination by limiting the continuity of the 

interface planes. However, failure often still results from interface failure. Toughening of the interface 

can be used to increase the durability and strength of composite structures by suppressing the 

delamination of the interface. Andersons (2004) discusses the role of interface damage propagation and 

toughness in overall composite failure and frames well the motivation for the approach that has been 

taken in this work. 

Toughening and other property enhancements of composite materials are typically implemented by 

modifying the bulk properties of the constituents, either the fiber or matrix materials. This often leads to 

difficulties in processing and higher material costs. Many composites consist of tows or yarns (thousands 

of individual fibers) that are either filament wound or processed into a fabric by weaving or braiding. The 

matrix material can be added to the tow or fabric before final processing, resulting in a prepreg material, 
NASA/TM—2014-216635 2 

or infused into the fiber material during final processing by a variety of methods. By using a direct 

electrospun deposition method to apply thermoplastic nanofiber to the surface of the tows, the tow-tow 

interface in the resulting composite can be modified while using otherwise conventional materials and 

handling processes. Other materials of interest could also be incorporated into the electrospun precursor. 

Approach 

Early work demonstrated the feasibility of directly depositing electrospun nanofibers onto continuous 

carbon fiber materials. Additional work consisted of three parts. The first was design and construction of 

a device for continuously depositing electrospun thermoplastic nanofiber onto larger quantities of carbon 

fiber prepreg tow. The second part involved the production of the modified tow material and fabrication 

of filament wound composite tubes. Tubes were fabricated at Pennsylvania State University with two 

fiber orientations using both the original tow and the modified material. The third part of the work 

involved characterization and testing of the fabricated tubes to evaluate the effects of the nanofiber 

toughening on mechanical properties. 

Electrospinning Method 

The electrospinning method is a process in which a precursor material is formed into a thin filament 

by electrostatic forces. The fibers produced can be on the nanometer scale. First, the fiber material is 

chosen and dissolved in a solvent. Typically, thermoplastic polymers are used. A high electric potential is 

then applied to the solution. The electrostatic forces stretch the solution into a Taylor cone which then 

forms a jet that elongates and solidifies as the solvent evaporates. The charged fibers are then collected on 

a grounded or oppositely charged target. As the fiber stretches it becomes unstable. This results in a 

random fiber placement. The solution can be handled in several different ways including passing the 

solution through a needle or porous foam, or by rotating a partially submerged roller in a bath of 

precursor solution. Figure 1 is a simple sketch of the electrospinning process using a needle. 

Figure 1.—Basic sketch of electrospinning process. 
NASA/TM—2014-216635 3 

Many different variables can affect the electrospun fiber. Some of these include solution viscosity, 

surface tension, solvent vapor pressure, ambient solvent vapor pressure, humidity, electric potential, 

polymer mechanical properties, solution conductivity, target and solution delivery geometry, temperature, 

and ambient gas flow conditions such as cross flow or turbulence. Bhattacharjee (2011) provides a 

comprehensive overview of the electrospinning process and the many variables involved.

Early Work 

Early work performed during a Glenn Research Center Innovation Fund Fast Track project in 2011 

demonstrated the feasibility of directly electrospinning fiber onto a carbon fiber tow. A solution of 

15 percent by weight polyethersulfone (PES) dissolved in dimethylformamide (DMF) was selected as the 

nanofiber precursor. PES was selected for this early work for its mechanical properties and availability. The 

solution was supplied to a circular array of eight stainless steel 16 gauge flat tip needles. Flow to the needles 

was controlled by manifold with a series of needle valves. A peristaltic pump was used to supply solution to 

the manifold. Polypropylene and silicone tubing were used for transport of the electrospinning precursor 

solution. Homaeigohar (2010) and Tang (2009) provide more information on electrospinning of PES. 

A potential of 20 kV was applied to the needles by charging the needle support ring. This ring was 

electrically isolated by suspending it from silicone tubing within a large fume hood. To create the coating 

on the carbon fiber tow, the electrospun nanofibers (still positively charged) were collected 

electrostatically on a grounded carbon fiber tow passing through the center of the needle array. The 

carbon tow was diverted through the needle array from the original path through a tow coating and 

winding machine (existing equipment that was borrowed for this test) using pulleys attached to the 

ventilation hood by magnets. The coated carbon fiber tow was slowly pulled through the electrospinning 

needle array and collected continuously on the winding machine’s rotating drum. 

Figure 2 (left) shows large clumps of nanofiber forming and depositing on the surface of the 

grounded carbon fiber tow running vertically through the center of the needle ring (indicated by red oval). 

The image on the right is the solution supply pump and manifold. 

Scanning electron microscopy (SEM) images of the resulting fibers on the surface of a carbon fiber 

tow at 500x (left) and 2000x (right) are shown in Figure 3. Typical diameters are in the 50 to 250 nm 

range. The beads observed in the SEM images are likely the result of either incomplete dissolution of the 

PES powder, too low of a dissolved polymer fraction in the solution, or both. Electrospinning with a 

solution having too low of a viscosity often results in electrospraying rather electrospinning which 

produces very small droplets. Initial attempts at obtaining the target 20 percent solution were unsuccessful 

due to solvent saturation and subsequent PES precipitation. Typically, PES would be very soluble in 

DMF, however the PES used in this project was obtained from a readily available sample batch that was 

chemically functionalized. 

Figure 2.—Nanofibers accumulating on the carbon fiber tow (left), polymer solution supply (right). 
NASA/TM—2014-216635 4 

Figure 3.—PES nanofiber on the surface of a carbon fiber tow at 500x (left) and 2000x (right). 

Figure 4.—Electrospun nanofiber deposition machine. 

The results of this early work were used in a proposal for additional funding under the ARMD 

Seedling Fund in 2012. The results of which will be discussed in the following sections. 

Electrospun Deposition Machine 

Following the successful demonstration of the deposition method, additional work was proposed and 

funded by NASA’s ARMD Seedling Fund. The first portion of this new work focused on the 

development of a machine to produce several thousand meters of nanofiber coated tow prepreg. The 

machine that was built consists of two main sections, the winding stage and the deposition chamber. 

Figure 4 shows an overview of the machine. 

 
NASA/TM—2014-216635 5 

Figure 5.—Carbon fiber wind/unwind stage. 

The wind/unwind stage holds two cardboard carrier spools. The uncoated spool is located on the right 

in Figure 5. A motor with shorted windings applies drag to the spool through a chain and sprocket (rear, 

not visible). The carbon tow passes between a pair of LED’s and a pair of photoresistors as it comes off 

the spool. A microcontroller monitors the voltage ratio on the photoresistors and provides control signals 

to the wind motor and the lateral stage motor. In this way, the entire stage is controlled to ensure the 

carbon tow is fed straight onto the pulleys, through the machine, and onto the wind spool with the same 

wind angle as the original spool. The cardboard carrier tubes are held in place using a pair of pipe test 

plugs on a length of all-thread; a pipe test plug consists of two opposed conical disks that when 

compressed expand a rubber ring into contact with the inner diameter of the tube. Flanged bearings hold 

the all-thread and tube assembly in place in slanted notches to allow the assembly to be easily removed 

and the spool quickly changed. 

The unwound tow then passes through the deposition chamber shown in Figure 6. The deposition 

chamber is a transparent plastic tube with plates on each end. Two fittings on each end plate provide 

purge air and allow the tow to pass into the chamber. An airlock is formed by using a “T” fitting with the 

intersecting side connected to the ventilation system. This way, the fiber can pass into the chamber and 

the chamber atmosphere can be controlled. The purge air forces the solvent vapors out of the chamber and 

into the ventilation system through the airlocks. Also, since the ventilation system operates at a higher 

flow rate than the purge air, solvent vapors do not enter the room and the room air does not enter the 

chamber. 

The polymer precursor solution is held in nine small bottles located at the bottom of the chamber. 

Each bottle was slightly pressurized (around 1 psi or 7 kPa) to force the solution up a dip tube and 

through a 22 gauge, 2 in. (50.8 mm) long stainless steel needle. A high strength rare earth magnet inside 

the chamber and a large fender washer located at the base of the needle was used to hold the needle with 

attached reservoir, tubing, and wiring in place. A high voltage power supply was used to apply an electric 

potential of between 20 and 25 kV to the needles. 
NASA/TM—2014-216635 6 

Figure 6.—Electrospun deposition chamber. 

Figure 7.—Contour plots of electric potential for the selected needle array on the inner chamber surface, 

scale bar shows voltage, 20 kV applied to the needles. 

The array of electrospinning needles was chosen to be located on the bottom of the chamber to 

minimize the possibility of droplets reaching the carbon tow. In earlier work in which the fiber was 

positioned vertically, large droplets of solution would deposit on the tow when the droplet at the opening 

of the needle would occasionally detach. An array consisting of nine needles provided a balance of 

electric field overlap and electrospun nanofiber production rate. COMSOL software was used to 

investigate a range of needle placements and needle numbers within the constraints of the chamber 

volume. Figure 7 shows the electric potential resulting from the selected array. 

The deposition chamber is an 18 in. (457 mm) OD acrylic tube with a 1/4 in. (6.35 mm) wall, 24 in. 

(609 mm) long. Polycarbonate end plates cover the ends and hold the ports for the purge air and airlock 

assemblies. Three rows of three needles each are supported by rare earth magnets and are positioned 

along the bottom, and 45up the circumference of the tube. The needles are spaced 12 cm apart along the 

axis of the tube and the middle needle is centered. The rare earth magnets are held in place with silicone 

adhesive. 
NASA/TM—2014-216635 7 

Nanofiber Precursor Material Preparation 

Nylon 11 was selected for electrospinning due to its toughness and chemical resistance to most 

solvents. Nylon particles have also been considered as a matrix toughener for epoxies, and the use of 

electrospun nylon nanofiber mats as an interleaving material is believed to have the potential to improve 

the in-plane properties by bonding with the resin and reinforcement. Results from electrospinning nylon 

11 and nylon 6 nanofibers have been reported, but attempts to dissolve nylon in low toxicity solvents with 

low boiling points have not been successful due to polyamide being amphiphilic, which often makes this 

material difficult to use during large batch processing. 

Initial experimental efforts included dissolving nylon 11 pellets in a variety of solvents, with a heated 

solution of benzyl alcohol being the only solvent where nylon 11 was completely soluble. Unfortunately, 

when the solution cooled to room temperature the nylon recrystallized and the mixture turned into a 

waxy-like consistency. Salts and other additives were incorporated into the solution during the mixing 

process as an attempt to reduce the hydrogen bonding and allow nylon 11 to remain suspended in solution 

at room temperature, but none of those methods worked. Eventually, a procedure was adopted from 

Behler et al., and a 2.5 wt% nylon 11 electrospinning solution was successfully prepared from a mixture 

of three parts dichloromethane and one part formic acid (Behler, 2007). 

Nanofiber Deposition Machine Operation 

Carbon fiber towpreg was purchased from TCR Composites (Ogden, UT). The towpreg is made with 

T700SC-12K carbon fiber tow and B-staged UF3325 epoxy. The full spool was mounted on the unwind 

stage using the removable holder and expansion pipe test fittings. The end of the towpreg was pulled 

slowly from the spool by hand and fed through the optical position sensor/controller and around the first 

two guide pulleys. The wind controller was then powered on to maintain the spool position while the tow 

was pulled through the deposition chamber, around the grounding side pulleys, through the chamber, 

around the exit pulleys and onto the winding spool. The free end was wrapped around the winding spool 

and under the next wind to hold it in place. 

The tubing connecting the dip tube and air supply were sealed to the caps of the solution reservoirs 

using tacky tape, typically used in vacuum bagging operations. The reservoirs were then filled and the 

needles, with attached fender washers, and air lines connected. The mounting magnets supported the 

reservoirs and tubing while positioning the spinning needle. The high voltage connection was attached 

using a wiring harness with nine electrical clips and the safety shield panels were put into place. 

The ventilation was continuously operating, however operation was checked before start up. Once the 

needles, reservoirs, and tow were in place, the purge air was adjusted to 6 CFM (170 l/min) and inward 

flow at the airlock opening was verified. Then the winding controller was started and the high voltage 

supply turned on and set to 22.5 kV. The winder was set to run at 10 rpm. The resulting material 

throughput was roughly 8.5 ft/min (2.59 m/min); the linear speed increases as the material on the wind 

spool gets thicker and the radius increases. The final step was to start the solution flow. The air pressure 

supply valve was opened with the regulator turned to zero and the vent valve open. The vent valve was 

slowly closed, ensuring that the regulator maintained zero pressure, then the pressure was slowly 

increased until solution was seen at the tips of the needles. The pressure required for sufficient flow was 

typically around 1 psi (7 kPa), and always less than 2 psi (14 kPa). The pressure was adjusted to maintain 

droplets at the tips of several needles while avoiding splattering or running of the solution down the 

needle. The voltage also needed to be adjusted slightly (always between 20 and 25 kV) to maintain at 

least three needles operating correctly. 

Due to slight variations in each needle and reservoir, stable and sufficient flow to all nine needles was 

not possible. During the first few minutes of operation, most needles would electrospin, producing large 

amounts of nanofiber that would clump before depositing on the carbon tow. This was followed shortly 

by plugging of several needles from insufficient solution supply. Increasing the supply pressure would 

result in splattering and solution streaming down some of the needles. Steady state operation of the 
NASA/TM—2014-216635 8 

machine would result soon after and the three electrospinning needles continued to function until 

reservoirs emptied, typically after 1 to 1.5 hours of operation. Each reservoir had a capacity of 30 mL. 

When the reservoir needed to be refilled, the solution air supply valve was first turned off, then the 

supply vent valve opened. The high voltage supply was then turned off and the winding controller turned 

off. The bottles were then unthreaded from the caps, filled, and replaced. The winding controller was then 

turned on, the high voltage supply turned on to the previous setting, the solution air supply turned on, and 

the vent valve slowly closed. Any adjustments would then be made to the voltage and solution air supply 

pressure to maintain stable operation of three needles. 

Further work is needed to improve the current system. One possible solution is to use one pressure 

regulator for each needle instead of the common supply so that each needle could be adjusted 

individually. Individual needle control was attempted by regulation the solution flow of each needle in the 

earlier work, but the conductivity of the solution made adjustment during operation impossible because 

the valves were all charged to high potential. The use of the air pressure flow control system allowed the 

flow to be safely adjusted during operation. Figure 8 is a video of nanofiber deposition during operation. 

The carbon fiber towpreg is visible in the center of the chamber and the electrospinning needles are at the 

bottom. Small clumps of nanofiber can be seen depositing on the carbon towpreg as it passes twice 

through the chamber. After several hours of operation, a cobweb like structure is visible at the top of the 

chamber. This was formed by nanofiber that drifted upward into the purge air streams entering from the 

sides at the top of the chamber. 

Four batches of nanofiber coated material were produced. The first trial batch was used to adjust 

operation of the machine. The following three batches, consisting of roughly 12,000 ft (3658 m) of coated 

material produced during over 20 hours of total operation, were used to fabricate the coupons for 

comparison with the uncoated material. 

Figure 8.—Video of nanofiber deposition during operation. 
NASA/TM—2014-216635 9 

Coupon Fabrication 

Coupons were fabricated by Pennsylvania State University (PSU) under the direction of Dr. Charles 

Bakis, Grant Number NNX12AR01A. This section discusses the tubes produced at PSU, manufacturing 

method, some information on quality and prediction of mechanical properties of the specimens using 

ply-level stress analysis. 

Baseline Tubes 

This section describes 90and 45baseline (unmodified) carbon/epoxy tubes made at Penn State for 

NASA’s testing program. 

Aluminum mandrels with hemispherical end domes were used for making the tubes. The mandrels 

having 48.26-mm (1.9 in.) outer diameter were coated with two coats of Monocoat E340 release agent. 

Each coat of release agent was baked at 121 °C (250 °F) for 1 hr. 

All the tubes were wound using a single pre-impregnated tow (towpreg). During the entire winding 

process, tow tension was kept at 22.2 N (5 lb) and temperature of the part was kept at 66 °C (150 °F) 

using a radiant heater coupled with an infrared thermometer and digital temperature controller. 

Photographs of the winding process are shown in Figure 9. 

In order to achieve a wall thickness near 2.5 mm (0.1 in.), hoop-wound tubes were made with 

nine coverages with a circumferential pattern (each stroke is a coverage with a circumferential pattern). 

Likewise, the 45° tubes were made with four helical pattern coverages (each coverage is a layer in a 

helical pattern). A helical pattern of five was used in 45° tubes—meaning there are five rhombic patterns 

around the circumference of the tube. After the layers were wound, Dunstone 220R brand shrink-tape 

(Charlotte, NC) was circumferentially wound onto the parts with a 50 percent overlap. Following the 

application of shrink-tape, the parts were placed stationary in an oven and cured at 143 °C (290 °F) for 

2 hr. A photograph of a cured part, prior to removing the shrink tape, is shown in Figure 10. 

After cure, the shrink tape was peeled off and the tubes were cut into specimens having 20.3 cm 

(8 in.) and 7.6 cm (3 in.) lengths for tension and compression testing, respectively. Photographs of 

compression specimens are shown in Figure 11. The shallow spiral imprint seen on the surface of both 

tubes is caused by the shrink tape. 

Longitudinal cross sections of the tubes were cut, lightly polished, and inspected in an optical 

microscope to check the quality of the specimens. Typical photomicrographs are shown in Figure 12. 

Nothing out of the ordinary was observed in these inspections. 

Figure 9.—Photographs of filament winding process, 45 (left) and 90 (right). 
NASA/TM—2014-216635 10 

Figure 10.—Photograph of cured hoop-wound tube 

wrapped in shrink tape. 

Figure 11.—Photographs of finished specimens, 45 (left) and 90 (right). 

Figure 12.—Photomicrographs of longitudinal cross-sections of specimens, 45 (left) and 90 (right). 
NASA/TM—2014-216635 11 

Significant variation in the wall thickness was observed in the 45specimens (Figure 13). No 

correlation was observed between the thickness variation and the location of the specimen on the mandrel, 

the position of the measurement on the helical winding pattern, or even the method of winding the shrink 

tape (i.e., by hand or using an apparatus that precisely controlled pitch and tension). Further investigation 

would therefore be required to determine the cause of the variation. It was decided to use these tubes for 

testing in spite of the thickness variation. The average difference between the thickest part of the wall and 

the thinnest part of wall was 0.25 mm in the 45specimens and 0.09 mm in the hoop-wound specimens. 

Appendix A summarizes wall thickness measurements made at the top and bottom ends of each specimen. 

Classical laminated plate theory (CLPT) was used to predict the mechanical properties of the tubes under 

the thin-wall assumption (i.e., the wall’s properties are calculated as if the material was a flat plate). This 

approximation is considered appropriate since the wall thickness is roughly a tenth of the tube radius. 

Since a full set of elastic and strength properties for the TCR towpreg could not be found, best-guess 

estimates for a T700/epoxy composite were employed for the calculations. The estimated ply properties 

are listed in Table 1 and the estimated properties for the [45]4 and [90]9 tubes are listed in Table 2 and 

Table 3, respectively. 

Figure 13.—End-view of a 45 specimen 

showing wall thickness variation. 

TABLE 1.—ESTIMATED PLY PROPERTIES FOR 

UNIDIRECTIONAL T700/EPOXY, USED FOR 

CLPT PREDICTIONS OF TUBE PROPERTIES 

Longitudinal modulus E1 (GPa) 153 

Transverse modulus E2 (GPa) 10.5 

Longitudinal poisson’s ratio 12 0.3 

Shear modulus G12 (GPa) 8.27 

Longitudinal tensile strength F1t (MPa) 2760 

Longitudinal compressive strength F1c (MPa) 781 

Transverse tensile strength F2t (MPa) 76.5 

Transverse compressive strength F2c (MPa) 233 

Shear strength F6 (MPa) 89.6 
NASA/TM—2014-216635 12 

TABLE 2.—ESTIMATED PROPERTIES OF 

[45]4 T700/EPOXY TUBES 

Nominal OD (mm) 52.95 

Nominal ID (mm) 48.26 

Area (mm2) 373.1 

Mom. inert (mm4) 239400 

Tensile strength (MPa) 

(kN) 

179 

66.9 

Compressive strength (MPa) 

(kN) 

176 

65.6 

Torsional strength (MPa) 

(N-m) 

290 

2620 

Axial modulus (GPa) 27.7 

Shear modulus (GPa) 39.6 

Axial poisson’s ratio 0.68 

TABLE 3.—ESTIMATED PROPERTIES OF 

[90]9 T700/EPOXY TUBES 

Nominal OD (mm) 52.77 

Nominal ID (mm) 48.26 

Area (mm2) 358.1 

Mom. inert (mm4) 228900 

Tensile strength (MPa) 

(kN) 

76.5 

27.4 

Compressive strength (MPa) 

(kN) 

233 

83.5 

Torsional strength (MPa) 

(N-m) 

89.6 

778 

Axial modulus (GPa) 10.5 

Shear modulus (GPa) 8.27 

Axial poisson’s ratio 0.021 

Modified Tubes With Preliminary Modified Tow 

This section summarizes the winding characteristics of a preliminary batch of coated carbon/epoxy 

towpreg received on June 13, 2013. The outermost tow was marked clearly and seemed to unspool 

without any problem at the beginning of the winding process. But after a few passes, it was realized that 

the tow was not unspooling freely. When the process was stopped and the spool was taken out of the 

winder, it was seen that the outermost fiber could not unspool freely due to the restriction of the fibers 

that are crossing over it, as shown in Figure 14. As the outermost tow was pulled during winding, it cut 

the fibers that are crossing over it causing the occurrence of open-ended fibers as seen in Figure 15. This 

problem became more severe as the tow was allowed to cut more fibers and the number of the open-ended 

fibers accumulated. The problem was solved by manually cutting all the fibers that were crossing over the 

outermost tow, and by removing all of the open-ended fibers. This same behavior was also observed 

during unwinding of the spool during deposition of the nanofiber and was likely more severe in the 

modified tow due to rewinding. Performing the nanofiber deposition in line with the resin impregnation 

process would reduce or eliminate this problem. 
NASA/TM—2014-216635 13 

Figure 14.—Fibers crossing over the outermost tow. 

Figure 15.—Open ended fibers. 
NASA/TM—2014-216635 14 

Another behavior that was observed was the presence of small-diameter white colored fibers that did 

not stick well to the tow. This was observed right after the tow came out of the spool. These fibers are 

clumps of nanofiber that were deposited on the tow. The rollers of the filament winder did not seem to 

aggravate this “shedding” behavior, so we do not believe that it is a major problem. The cutting of tow 

required by the splitting behavior shown in Figure 15 is likely to be problematic for winding full mandrels 

in the future. This is likely due to handling of the tow either in the initial tow impregnation, shipping, or 

unwinding for nanofiber deposition as this was observed to a lesser extent during deposition unwinding. 

Some parts of the coated carbon/epoxy towpreg contained dark yellow colored regions, as seen in 

Figure 16. This is due to clumping of the nanofibers during the deposition process. The first few meters of 

the unspooled towpreg seemed to have a smaller and more variable tow width compared to the rest of the 

spool and the previously sent non-coated carbon/epoxy towpreg. After the first few meters, the tow width 

was consistent. It would be best if the cause of the tow width variation were eliminated so that the quality 

of the manufactured parts can be maximized. Filament winding relies on a consistent tow width for an 

optimal, homogeneous microstructure inside the composite. 

A 90compression specimen was wound with the limited amount of coated towpreg provided by 

NASA in this batch. Figure 17 shows optical microscope pictures of a polished cross-section, with the 

plane of the cut perpendicular to the fibers. At these magnifications, it is not possible to see the 

electrospun fibers in the matrix. 

Figure 16.—Yellow colored regions on the towpreg, highlighted with arrows. 
NASA/TM—2014-216635 15 

Figure 17.—Optical microscope images of polished cross-section from 90specimen, sectioned 

perpendicular to fibers, higher magnification from top left, to top right, to bottom. 

TABLE 4.—SUMMARY OF BATCHES OF TOWPREG 

Batch no. Date received at PSU

2 June 26, 2013 

3 June 27, 2013 

4 July 11, 2013 

Modified Tubes With Production Modified Tow 

This section summarizes the 90and 45modified (nanofiber-coated) carbon/epoxy tubes made at 

Penn State for NASA’s testing program. The coated tow was received in three batches (Table 4). 

Batch 1, received on June 13, 2013 was used to do a 90trial wind, as reported in the previous 

section. It was mentioned that loose (broken) fibers caused unspooling problems, which necessitated 

occasionally cutting off layers of fiber from the spool while winding. Batches 2, 3, and 4—the subject of 

the present section—did not have the unspooling problem seen in Batch 1. However, a new problem was 

observed which may have escaped detection in Batch 1. The new problem is the folding of the tow onto 

itself across the width, resulting in variable tow width. This problem is illustrated in a photograph of a 

45tube in Figure 18. The gap is shown clearly because of the contrasting color of the mandrel. While 

the gaps in the first layer wound onto the heated mandrel could be “fixed” to some degree by spreading 

the warmed tow manually, in the subsequent layers it was not possible to see the gaps by eye. The folds 

were visible in the towpreg as it came off the spool. This phenomenon was not seen in the uncoated 

towpreg. 
NASA/TM—2014-216635 16 

Figure 18.—Winding of a 45tube showing a folded-over tow and non-uniform 

tow width. 

Figure 19.—Void shown in a 45tube (left), higher magnification of the region around the same void (right). 

Some voids were observed in the 45tubes (Figure 19), which are believed to be related to the 

folded tows and resulting gaps that formed between tows as they were deposited onto the mandrel. During 

the 90winding, similar tow width problems were observed during winding, but large voids were not 

detected in the cured tubes. It is believed that the 90layers can “nest” better among themselves, in 

comparison to the 45tubes, thus eliminating voids due to fiber compaction. 

Variation in the wall thickness was observed in the tubes made with coated towpreg, as was the case 

with the unmodified material. The average difference between the thickest part of the wall and the 

thinnest part of wall was 0.23 mm in the 45specimens and 0.09 mm in the hoop-wound specimens. 

Appendix B summarizes wall thickness measurements made at the top and bottom ends of each specimen. 

In comparing the measurements in Appendix B with those for the tubes made with uncoated towpreg 

(Appendix A), it is seen that the coating process adds a few hundredths of a millimeter to the wall 

thickness of the tubes. The variability in wall thickness was unaffected, however. 
NASA/TM—2014-216635 17 

Testing and Characterization 

The materials were characterized at several steps during this work. Preliminary microscopy was used 

to check the suitability of the electrospinning precursor solutions but is not included. Scanning electron 

microscopy (SEM) was used to image the nanofiber after it was deposited on the carbon tow and after the 

tubes specimens were fabricated. Finally, mechanical tests were performed on the 45 and 90 specimens 

that included tension and compression. This section will cover these results. 

Deposited Nanofiber

Scanning electron microscopy (SEM) was used to image the as-deposited nanofiber on the surface of 

the carbon fiber towpreg and in a cut sample of cured composite. The expected random mesh was 

observed on the towpreg surface. Measurements of the nanofiber diameters ranged from 100 to 300 nm 

before and after composite fabrication. Figure 20 shows the deposited nanofiber mesh pressed into the 

resin on the surface of the carbon fiber towpreg. SEM images of the nanofiber in cured composite are 

included in Appendix D. The carbon fiber is not visible. 

Figure 21 shows the nanofiber mesh spanning ridges on the surface of the carbon fiber towpreg (left) 

and a close up showing the resin wicking onto the nanofiber mesh (right). 

Figure 20.—SEM images showing the electrospun nanofiber pressed into the resin on the towpreg surface after 

deposition (left), increased magnification (right). 

Figure 21.—SEM images showing the nanofiber mesh spanning surface ridges on the carbon fiber towpreg (left) and 

a close up showing the resin wicking onto the nanofiber mesh (right). 
NASA/TM—2014-216635 18 

Mechanical Testing 

Mechanical strength tests were performed using a screw-driven electromechanical load frame, Instron 

model 1125 with an MTS Sintech Upgrade Package. The machine was equipped with MTS 647 hydraulic 

wedge grips. For the tension and compression tests, a displacement rate of 0.05 in./min was used. The 

tensile tube coupons were bonded into stainless steel end caps using Loctite Hysol E-60HP 2 part epoxy. 

Five sets of end caps were fabricated so that all specimens of each set could be bonded and tested 

together. Following each test set, the center section of the tube was cut out and the end caps with the ends 

still bonded in were burned out at 450 °C for 6 to 8 hr and the remaining residue removed by abrasive grit 

blasting. A complete tensile specimen is shown in Figure 22 (top) and a section view of the assembly 

(bottom). The ends of the assembly are threaded to interface with calibration hardware adapters that were 

used to interface with the wedge grips in the machine. 

The 45tensile specimens deformed significantly (radial contraction) following the onset of 

nonlinearity above 8000 lb (35.6 kN) load (Appendix C). Ultimate failure was characterized by a peak in 

load followed by an extended unloading (negative load/displacement curve slope) resulting from 

progressive delamination of the tows. The 90tensile specimens failed at the end cap bond line at 

relatively low load with no nonlinear deformation as anticipated. Figure 23 shows a 45tensile 

specimen after failure, still mounted in the load frame. 

The ends of the compression coupons were cut flat and parallel using a precision diamond saw. Steel 

end plates with a shallow channel (0.125 in. deep by 0.125 in. wide with a mean diameter of 1.985 in. or 

3.175 mm deep by 3.175 mm wide with a mean diameter of 50.419 mm) were used on the ends of the 

coupons. The end plates were not bonded to the specimens. The coupon with loose fitting end plates was 

placed in the test machine between flat compression platens. Figure 24 shows the compression test end 

plates and a 45 specimen. 

Figure 22.—Finished tensile specimen (top) and section view of assembly (bottom). 
NASA/TM—2014-216635 19 

Figure 23.—Failed 45 tension specimen still mounted in test machine. 

Figure 24.—Compression test end plates and 45 specimen. 

 
NASA/TM—2014-216635 20 

Figure 25.—Examples of compression specimen failures, 45 (left) and 90 (right). 

The ±45compression specimens show a significant change in stiffness (softening) at higher load that 

was associated with severe barreling of the tube. The final failure resulted in fractures running along the 

45directions of the fibers, often near one of the end plates. The 90tubes showed significantly less nonlinearity. The compression specimens typically failed toward the middle of the specimen and resulted in 

broken hoop direction fibers and a local ring of compression damage. Examples of the failure of 45 and 

90 compression test specimens are shown in Figure 25. 

The load-displacement curves for the coupon tests are shown in Appendix C. The peak load for each 

specimen was recorded and the average and standard deviations computed. Figure 26 shows the measured 

tension strengths with error bars representing two times the standard deviation. The baseline values are 

shown in blue and the modified tube values shown in red. The numbers in the bottom of each bar indicate 

the number of specimens used in the computations. 

Figure 27 shows the measured compression strengths with error bars representing two times the 

standard deviation. The baseline values are shown in blue and the modified tube values shown in red. The 

numbers in the bottom of each bar indicate the number of specimens used in the computations. The 45

tension tubes with the added nanofiber toughener shows an increase in average strength of 11.6 percent 

over baseline with two times the standard deviation of baseline and modified set not showing an overlap. 

The average strengths of the remaining modified test sets show no significant difference as compared to 

the baselines when the standard deviation overlaps are considered. This is true not only for two times 

standard deviation but one standard deviation as well. This is because the main driver for ultimate failure 

of the 45° tension tube is interlaminar shearing of the tow to tow interface, whereas the other specimens 

undergo different failure mechanisms including in-plane shear and transverse tension that are less affected 

by the nanofiber toughener. 

The lack of change in the 90tension specimens may imply poor bonding of the nanofiber to the 

matrix material or that an insignificant quantity is present to appreciably affect the transverse tensile 

strength of the composite. Additional testing is needed to further investigate the adhesion of the epoxy 

matrix to the polymer nanofiber. 
NASA/TM—2014-216635 21 

Figure 26.—Average peak stress for tension specimens, error bars represent two times 

standard deviation, the number of specimens appear in the bottom of each bar, baseline 

(blue) and modified (red). 

Figure 27.—Average peak stress for compression specimens, error bars represent two times 

standard deviation, the number of specimens appear in the bottom of each bar, baseline 

(blue) and modified (red).
NASA/TM—2014-216635 22 

Summary 

The use of direct electrospun deposition of thermoplastic nanofibers on continuous carbon material 

for toughening of composites was demonstrated. The process was first demonstrated at small scale and 

then using a larger continuous lab scale process that resulted in the production of roughly 12,000 ft 

(3658 m) of nanofiber coated towpreg. The coated material was used to fabricate tension and compression 

coupons with 45and hoop wound 90architectures. Aside from the 45 tension coupons, no change 

was observed in the strength measurements. The 45 tension coupons however, showed an increase of 

11.6 percent in tensile strength compared to the baseline material. No overlap in two times the standard 

deviation of each test set was observed. More work is needed to improve the deposition process. 

Investigation of the compatibility and suitability of different thermoplastic nanofiber materials could 

result in more significant improvements in interface toughness and therefore composite strength. 

 
NASA/TM—2014-216635 23 

Appendix A.—Wall Thickness Measurements of Baseline Tubes 

Specimen identification notation: 

45 (or 90) Winding angle 

UM Unmodified prepreg (no nanofibers) 

T Tension test specimen 

C Compression test specimen 

TABLE 5.—UNMODIFIED [45]4 TUBES 

Specimen ID Minimum 

(mm) 

Maximum 

(mm) 

Max-Min 

(mm) 

45-UM-T1 Top 2.24 2.50 0.26 

Bottom 2.22 2.40 0.18 

45-UM-T2 Top 2.16 2.54 0.38 

Bottom 2.26 2.52 0.26 

45-UM-T3 Top 2.26 2.48 0.22 

Bottom 2.16 2.50 0.34 

45-UM-T4 Top 2.14 2.50 0.36 

Bottom 2.14 2.50 0.36 

45-UM-T5 Top 2.20 2.46 0.26 

Bottom 2.02 2.52 0.50 

45-UM-C1 Top 2.28 2.48 0.20 

Bottom 2.28 2.44 0.16 

45-UM-C2 Top 2.30 2.48 0.18 

Bottom 2.26 2.46 0.20 

45-UM-C3 Top 2.26 2.42 0.16 

Bottom 2.26 2.46 0.20 

45-UM-C4 Top 2.28 2.48 0.20 

Bottom 2.30 2.50 0.20 

45-UM-C5 Top 2.12 2.48 0.36 

Bottom 2.18 2.50 0.32 

45-UM-C6 Top 2.26 2.44 0.18 

Bottom 2.32 2.46 0.14 

45-UM-C7 Top 2.26 2.44 0.18 

Bottom 2.24 2.42 0.18 

45-UM-C8 Top 2.20 2.40 0.20 

Bottom 2.22 2.42 0.20 

45-UM-C9 Top 2.26 2.48 0.22 

Bottom 2.24 2.48 0.24 

45-UM-C10 Top 2.14 2.52 0.38 

Bottom 2.16 2.50 0.34 

 Avg. 2.22 2.47 0.25 
NASA/TM—2014-216635 24 

TABLE 6.—UNMODIFIED [90]9 TUBES 

Specimen ID Minimum 

(mm) 

Maximum 

(mm) 

Max-Min 

(mm) 

90-UM-T1 Top 2.36 2.42 0.06 

Bottom 2.32 2.42 0.10 

90-UM-T2 Top 2.28 2.36 0.08 

Bottom 2.22 2.34 0.12 

90-UM-T3 Top 2.22 2.34 0.12 

Bottom 2.22 2.32 0.10 

90-UM-T4 Top 2.16 2.28 0.12 

Bottom 2.16 2.28 0.12 

90-UM-T5 Top 2.18 2.26 0.08 

Bottom 2.16 2.26 0.10 

90-UM-C1 Top 2.20 2.30 0.10 

Bottom 2.20 2.28 0.08 

90-UM-C2 Top 2.20 2.28 0.08 

Bottom 2.22 2.26 0.04 

90-UM-C3 Top 2.26 2.34 0.08 

Bottom 2.26 2.32 0.06 

90-UM-C4 Top 2.18 2.26 0.08 

Bottom 2.18 2.28 0.10 

90-UM-C5 Top 2.22 2.30 0.08 

Bottom 2.22 2.34 0.12 

90-UM-C6 Top 2.28 2.38 0.10 

Bottom 2.24 2.36 0.12 

90-UM-C7 Top 2.18 2.26 0.08 

Bottom 2.16 2.24 0.08 

90-UM-C8 Top 2.18 2.24 0.06 

Bottom 2.18 2.24 0.06 

90-UM-C9 Top 2.18 2.24 0.06 

Bottom 2.18 2.24 0.06 

90-UM-C10 Top 2.20 2.28 0.08 

Bottom 2.20 2.26 0.06 

 Avg. 2.21 2.30 0.09 

 
NASA/TM—2014-216635 25 

Appendix B.—Wall Thickness Measurements of Modified Tubes 

Specimen identification notation: 

45 (or 90) Winding angle 

M Modified towpreg (nanofiber coating) 

T Tension test specimen 

C Compression test specimen 

TABLE 7.—MODIFIED [±45]4 TUBES 

Specimen ID Minimum 

(mm) 

Maximum 

(mm) 

Max-Min 

(mm) 

45-M-T1 Top 2.39 2.62 0.23 

Bottom 2.26 2.44 0.18 

45-M-T2 Top 2.26 2.49 0.23 

Bottom 2.26 2.51 0.25 

45-M-T3 Top 2.26 2.51 0.25 

Bottom 2.24 2.41 0.17 

45-M-T4 Top 2.36 2.51 0.15 

Bottom 2.24 2.57 0.33 

45-M-T5 Top 2.26 2.57 0.31 

Bottom 2.29 2.51 0.22 

45-M-C1 Top 2.29 2.41 0.12 

Bottom 2.26 2.46 0.2 

45-M-C2 Top 2.24 2.54 0.3 

Bottom 2.24 2.51 0.27 

45-M-C3 Top 2.26 2.46 0.2 

Bottom 2.24 2.51 0.27 

45-M-C4 Top 2.31 2.49 0.18 

Bottom 2.29 2.49 0.2 

45-M-C5 Top 2.26 2.59 0.33 

Bottom 2.31 2.57 0.26 

45-M-C6 Top 2.29 2.46 0.17 

Bottom 2.24 2.49 0.25 

45-M-C7 Top 2.24 2.46 0.22 

Bottom 2.26 2.46 0.2 

45-M-C8 Top 2.21 2.46 0.25 

Bottom 2.24 2.49 0.25 

45-M-C9 Top 2.24 2.51 0.27 

Bottom 2.21 2.49 0.28 

45-M-C10 Top 2.24 2.49 0.25 

Bottom 2.24 2.46 0.22 

 AVG 2.26 2.50 0.23 

 
NASA/TM—2014-216635 26 

TABLE 8.—[90]9 TUBES 

Specimen ID Minimum 

(mm) 

Maximum 

(mm) 

Max-Min 

(mm) 

90-M-T1 Top 2.31 2.46 0.15 

Bottom 2.29 2.41 0.12 

90-M-T2 Top 2.26 2.41 0.15 

Bottom 2.26 2.41 0.15 

90-M-T3 Top 2.29 2.41 0.12 

Bottom 2.29 2.41 0.12 

90-M-T4 Top 2.31 2.41 0.1 

Bottom 2.31 2.41 0.1 

90-M-T5 Top 2.31 2.41 0.1 

Bottom 2.29 2.41 0.12 

90-M-C1 Top 2.24 2.31 0.07 

Bottom 2.26 2.36 0.1 

90-M-C2 Top 2.34 2.36 0.02 

Bottom 2.26 2.34 0.08 

90-M-C3 Top 2.24 2.34 0.1 

Bottom 2.24 2.36 0.12 

90-M-C4 Top 2.29 2.41 0.12 

Bottom 2.26 2.41 0.15 

90-M-C5 Top 2.29 2.36 0.07 

Bottom 2.26 2.31 0.05 

90-M-C6 Top 2.34 2.39 0.05 

Bottom 2.26 2.31 0.05 

90-M-C7 Top 2.24 2.36 0.12 

Bottom 2.31 2.36 0.05 

90-M-C8 Top 2.34 2.39 0.05 

Bottom 2.34 2.39 0.05 

90-M-C9 Top 2.36 2.41 0.05 

Bottom 2.34 2.39 0.05 

90-M-C10 Top 2.31 2.36 0.05 

Bottom 2.39 2.44 0.05 

 AVG 2.29 2.38 0.09 

 
NASA/TM—2014-216635 27 

Appendix C.—Load Displacement Curves of Test Coupons 

Figure 28.—Load vs. displacement curves for all tubes with the 45wind angle loaded in 

tension. 

Figure 29.—Load vs. displacement curves for all tubes with the 90wind angle loaded in 

tension. 
NASA/TM—2014-216635 28 

Figure 30.—Load vs. displacement curves for all tubes with the 45wind angle loaded in 

compression. 

Figure 31.—Load vs. displacement curves for all tubes with the 90wind angle loaded in 

compression. 
NASA/TM—2014-216635 29 

Appendix D.—Scanning Electron Microscopy of 

Electrospun Nanofibers in Cured Composite 

Figure 32.—Scanning electron microscopy image of electrospun nanofibers protruding 

from cut surface of cured composite. 

Figure 33.—Scanning electron microscopy image of electrospun nanofiber protruding from 

cut surface of cured composite, diameter measured at 257 nm. 
NASA/TM—2014-216635 30 

Figure 34.—Scanning electron microscopy image of electrospun nanofibers protruding 

from cut surface of cured composite, diameters measured at 90 and 151 nm. 

Figure 35.—Scanning electron microscopy image of electrospun nanofiber protruding 

from cut surface of cured composite, diameter measured at 141 nm. 
NASA/TM—2014-216635 31 

Figure 36.—Scanning electron microscopy image of electrospun nanofiber protruding from 

cut surface of cured composite, diameter measured at 156 nm. 

 
NASA/TM—2014-216635 32 

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