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European Journal of Applied Sciences – Vol.10, No.5

Publication Date: October 25, 2022

DOI:10.14738/aivp.105.13312. Greeley, D., Gritton, J., Minneci, R., Plymill, A., & Bhat, G. (2022). Structure and Properties of PLA Nanocomposite Feedstock for

Additive Manufacturing. European Journal of Applied Sciences, 10(5). 575-587.

Services for Science and Education – United Kingdom

Structure and Properties of PLA Nanocomposite Feedstock for

Additive Manufacturing

Duncan Greeley

The University of Tennessee, Knoxville

Jack Gritton

The University of Tennessee, Knoxville

Robert Minneci

The University of Tennessee, Knoxville

Austin Plymill

The University of Tennessee, Knoxville

Gajanan Bhat

University of Georgia, Athens, GA

ABSTRACT

Additive manufacturing is continuing to grow with a large potential for

replacement of traditionally manufactured parts, part repair, and prototype

development due to complexity of allowable part geometry and low raw material

use. However mechanical properties of parts processed through additive

manufacturing typically suffer in comparison. The goal of this study was to

develop an improved and sustainable feedstock material for additive

manufacturing through reinforcement of polylactic acid with nanoparticles such

as graphene and multiwalled carbon nanotubes. Composites with loadings of 0.1,

0.2, and 0.5 wt% of each reinforcement were extruded to form filament feedstock,

and tensile and impact specimens were printed using a Lulzbot Mini according to

ASTM D638 and D256 test methods. Mechanical properties were evaluated

through tensile and impact testing, while fracture surfaces were analyzed using a

scanning electron microscope. Thermal properties of the feedstock material and

post-printed material were analyzed with differential scanning calorimetry.

Reinforcements led to a moderate increase in mechanical properties with the 0.2

wt% loading of graphene showing a 47% increase in tensile strength, a 17%

increase in modulus, and 12% increase in energy absorbed upon fracture. The 0.1

wt% loading of MWCNT had respective increases of 41%, 16%, and 9%, in tensile

strength, modulus and fracture energy. Adding small amount of nanoparticles

during processing is easy to accomplish and the improvement in performance may

be justified for many applications.

Key Words: PLA, Additiver Manufacturing, Nanocomposites, Graphene, Carbon

Nanotubes, Mechanical Properties, Thermal Properties

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European Journal of Applied Sciences (EJAS) Vol.10, Issue 5, October-2022

Services for Science and Education – United Kingdom

INTRODUCTION

Additive manufacturing (AM) has been increasingly implemented over the past several years

as an efficient means to prototype and produce components with a high degree of shape

complexity [1-3]. When compared to traditional subtractive processes, AM allows for a

reduction in raw material use which leads to significant cost savings through a lower buy-to- fly ratio (weight ratio between raw material and final part). However as AM is still a relatively

new process, parts manufactured in this method tend to have disadvantages compared to

parts manufactured through traditional methods, namely inferior mechanical properties, and

property anisotropy due to the bonding strength between layers and porosity in the final

structure [4]. In this study, additive processing of polymeric materials as opposed to metals

was investigated, as they offer significant flexibility in feedstock modification through

additives or reinforcements. One of the most promising methods of improving the mechanical

properties of polymeric parts involves reinforcement of the polymer resin with stronger,

lightweight materials dispersed throughout the matrix. For instance, high strength high

modulus fibers such as carbon fiber are often used to increase the strength and stiffness of

several types of polymers [5]. The improvements of these properties in the final product

typically follows a simple rule of mixtures for uniaxially oriented fibers such as in the case for

the tensile modulus:

(1) E = (1-Vf)Em + VfEf

where E is the longitudinal modulus, Vf is the volume fraction of the fiber, Em is the modulus of

the matrix, and Ef is the modulus of the fiber [6].

As fused deposition modeling (FDM) is one of the most common polymer additive processes,

it was utilized for this study to allow for a wider applicability of results to current industrial

applications. In FDM, polymeric filament is fed by a tractor wheel arrangement into a heated

extruder head which deposits the material by tracing a sliced layer of a CAD model [4]. During

printing, the extruder head typically moves along an XY plane to trace a layer, followed by a

change in the Z direction by dropping the bed or raising the head before printing of the

subsequent layer. One of the main reasons for the common use of AM is the wide material

customization offerings due to the ability to fine tune a variety of build parameters. These

include but are not limited to layer thickness, layer orientation, raster pattern, raster angle,

raster width, raster air gap, bed temperature, extrusion temperature, extrusion speed, nozzle

shape and diameter, support pattern, contour width, contour air gap, and number of contours

[44].

One of the main polymeric materials of interest for AM is polylactic acid (PLA) as it is both

biodegradable and biocompatible[xx]. These properties allow it to be used for biomedical

components such as bone replacement tissue scaffolds that benefit from intricate

customizable patterns [1]. Serra et al. recently were able to create high-resolution PLA-based

additively manufactured tissue scaffolds [3]. The improvement of the mechanical properties

of these biomedical components could allow for both improved durability and enhanced

performance of the parts. A promising route for improving the mechanical properties of PLA

lies in carbon-based reinforcement. Kuan et al. demonstrated that the strength of PLA could

be greatly increased with small amounts of multi-walled carbon nanotubes (MWCNT)

dispersed in the polymer matrix [4]. Similarly, Pinto et al. showed that the strength and

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Greeley, D., Gritton, J., Minneci, R., Plymill, A., & Bhat, G. (2022). Structure and Properties of PLA Nanocomposite Feedstock for Additive

Manufacturing. European Journal of Applied Sciences, 10(5). 575-587.

URL: http://dx.doi.org/10.14738/aivp.105.13312

modulus of PLA could be improved with the addition of graphene [5]. PLA is also sensitive to

moisture during processing, and must be dried prior to use otherwise flow instabilities will

occur upon heating. The particular PLA resin used in this project was the amorphous

Natureworks 4060D. 4060D is traditionally used for thin film applications, however the

excellent rheological properties of this grade makes it optimal for AM applications.

Mechanical properties can also be increased through adjustment of the FDM processing

parameters such as build speed, build direction in relation to the part orientation, layer

thickness, and fill pattern. Halil et al. investigated chopped carbon fiber reinforcement of a

similar FDM feedstock, acrylonitrile-butadiene-styrene (ABS), and discovered that while

mechanical properties increased with increased fiber loadings, void content within the bead

correspondingly increased due to poor interfacial adhesion [6]. Rezayat et al. modeled the

relationship between the build parameter structure and the mechanical properties in FDM of

ABS, and found that adjusting the air gap in between rasters from negative to positive resulted

in a change in the load transfer mechanism from the raster to the contour of the part, implying

that statistically relevant tensile results can only be obtained if the build parameters are

carefully selected [7]. Ahn further investigated the effect of varying process parameters on the

anisotropy of the material properties in FDM, and emphasized that for max tensile loads, the

load direction should align with the reinforcement orientation [8]. Torres et al. tested

torsional mechanical properties of PLA in FDM and concluded that near-bulk PLA properties

could be obtained through specific parameter optimization, however it was also observed that

while post-process heat treatments slightly increased strength, ductility decreased

dramatically [9].

The goal of this research was to fabricate and test PLA additive feedstock reinforced

separately with MWCNT and graphene at 0.1, 0.2 and 0.5 wt% loadings. The effect of varied

reinforcement loadings on the structure and properties of the feedstocks were investigated

using a wide range of characterization techniques.

EXPERIMENTAL

Commercial grade 4060D PLA resin pellets were provided by NatureWorks LLC (Minnetonka,

MN) for this project. Graphene was provided by Celtig LLC (Knoxville, TN) and multiwall

carbon nanotubes were purchased from SES Research (Houston, TX). Sample fabrication and

testing were done at the University of Tennessee, Knoxville. PLA was heated at 80°C overnight

to dry the pellets in a Novatec dessicator. Two masterbatches of PLA composites were

produced via dry mixing of 400g PLA and 600g PLA with 1 wt% MWCNT and 1 wt% graphene

respectively. The mixtures were extruded at 200°C from a Haake twin screw extruder at a

screw speed of 23 rotations per minute for MWCNT composite and 25 rotations per minute

for the graphene composite, and both filaments were fed directly into a water bath. The Haake

screw was used for its high shear rate leading to achieve effective mixing of the nanoparticles.

The master batches were dried overnight and pelletized in a laboratory mill for later use.

The Filabot EX2 single screw extruder was utilized in order to fabricate the AM filament for

the 3D printers, but the filament winder proved to be difficult to use properly and was not

used. The filament needed to be roughly three millimeters in diameter for proper printing

which was the maximum available die diameter, so significant process control had to be