Brandon Spradlin

Chemistry 450 Dr. Neal

November 12, 2014

3D Printing Composite Plastic

Abstract: 3D printing is an additive manufacturing process where small, thin layers of plastic

are successively added one on top of one another. Acrylonitrile- Butadiene- Styrene (ABS) is a

common plastic that is used in 3D printers. Although current printing techniques are suitable for

toys and small-scale applications, the strength of the printed material must be increased to adapt

to an industrial scale. One analysis has shown that the point of contact between two layers is the

weakest part of the structure. A method that is being explored is mixing carbon nanotubes into

the plastic before it is printed. Once it is heated and printed out, the carbon nanotubes should mix

between the new and old layers and strengthen the bond at that interface. Because 3D printing is

a continuous process, the ABS- nanotube mixture must be as homogeneously mixed as possible.

Direct mixing and solution mixing are two techniques that are being used to mix the carbon

nanotubes with the ABS.

Introduction

Today, plastic is absolutely everywhere and it has a plethora of uses. It is used as water

bottles, toys, bullet proof glass, flame retardant materials, and so on. Each type of plastic, or

polymer, performs differently under different conditions, which gives rise to how we use it. The

type of polymer that can be re-melted and molded again is referred to as a thermoplastic, and

those that cannot be re-melted and molded are called thermosets. ABS (acrylonitrile-butadiene-

styrene) is a very common thermoplastic that is used in our everyday lives from cups, to

children’s toys. The advantage of thermoplastics is that an object can be made by vacuum

forming the plastic over the desired mold when it is warmed up, or, by heating the plastic and

letting it flow into a mold. The disadvantage, however, is that a new mold must be created for

each new type of object that needs to be made. This is where extrusion 3D printers really gain a

foothold over conventional plastic molding. An extrusion 3D printer is basically a machine that

adds a small bead of plastic one layer at a time. The plastic is heated into a pliable form, then

added successively one layer on another. The advantage is that there is no need for a mold, and

new objects can quickly be made. Computer aided design (CAD) software is used to create the

object on a computer, which can be transferred to the printing software. Instead of creating a

mold, then melting the plastic and molding it, a 3D printer melts the plastic and ‘prints’ whatever

is drawn in the CAD software.

The 3D printer, as seen in the diagram below, consists of a heated metal extruder (nozzle)

that can move in the X and Y directions, a heated bed that can move vertically (Z direction), and

the desired plastic (called filament; it resembles grass-trimmer wire).

 

3D Printer 1: A Solidoodle Workbench 3D Printer

Initially, the extruder must be heated to around 210ºC for ABS, and the bed to around

100ºC. The bed is heated to make sure the plastic adheres slightly to keep the object in place

while each layer is added. Once they are heated, the extruder slides over the top of the bed and

begins extruding, or printing, the pliable plastic wire. The extruder moves in the XY direction,

which creates one layer of the object. Once the first layer is applied, the bed moves down slightly

(Z direction), and the next layer is printed until the object is made.

When each new layer of plastic is added it binds to the layer underneath while it cools

and hardens. The layers are visible to the naked eye, and this interface between layers is the

weakest point in the structure. To get an idea of the interface, picture two pairs of spaghetti

noodles laying on top of one another (Figure1 below)- when the noodles dry, they will adhere to

Extruder head (filament enters through the top)

Extruder

Print Bed

each other and form a solid structure of four noodles They are weaker where they are joined

together, and stronger in the center of each solid noodle.

 

Figure 1: Horizontal view of 4 plastic layers with arrows indicating the strongest and weakest points.

This interface becomes problematic when trying to increase the scale of the printed

objects, and structural integrity must be taken into consideration. Increasing the temperature of

the extruder is an option, however the quality of the printed object diminishes. One solution that

is being explored is the implementation of multi-walled carbon nanotubes (CNTs) into the ABS

plastic filaments. The idea is that the CNTs will mix at the interface of the plastic layers, and

create a ‘peg’ system that will lock the layers together.

Weak

Strong

Figure 2: Perpendicular view of two plastic layers. The lines connecting the two layers represent the carbon nanotubes that create the peg system.

Results/Discussion

Multi-walled carbon nanotubes have great stiffness and strength,1 which, when mixed

with ABS plastic (CNT composite), will increase the stiffness and strength of the composite as

well. CNT (seen in Figure 3 below) are nano-scale sheets of graphene that are covalently bonded

to form tubes.

 

The structural integrity of the CNT is enhanced due to the

large localization of covalent bonds throughout the tube.2

Different amounts of CNT are mixed into ABS plastic to

generate a variety of different CNT composites.

Composites consisting of 1%, 2%, 4% and 8% weight

CNT have been created to enhance the structural

properties of the object.3

Due to the nature of 3D printing, the problem with these composites is ensuring the CNT

is dispersed homogeneously throughout the filament, and the filament must retain the ability to

be heated and extruded through the nozzle. The composite filament can currently be made using

two techniques. The first is simply grinding pure ABS into a fine powder and mixing in the

desired amount of CNT. The mixture is blended until, ideally, the CNT is dispersed evenly. It is

then heated and extruded through a filament extruder to create a filament that is 1.75mm in                                                                                                                1 http://www.sigmaaldrich.com/technical-documents/articles/materials-science/single-double-2 Cumings, J.; Zettl, A. (2000). "Low-Friction Nanoscale Linear Bearing Realized from Multiwall Carbon Nanotubes". Science 289 (5479): 602–604. 3 Mari, D. and R. Schaller (2009). "Mechanical spectroscopy in carbon nanotube reinforced ABS." Mater. Sci. Eng., A A521-A522: 255-258.  

Figure 3: Representation of a carbon nanotube.

diameter. The other technique is to dissolve pure ABS in solvent and add the CNT. Continuous

stirring is expected to keep the mixture homogeneous. The ABS is then ‘crashed out’ and,

ideally, encapsulates the CNT. After drying, the mixture is heated and made into filament using

the filament extruder in the same way as above. The problem with both techniques is there is no

way to fully determine that the mixture is homogeneously mixed. The ideal composite would

have a certain amount of CNT per micrometer (or smaller) of ABS, but that level of precision

has not yet been established.

Dynamic mechanical analysis is the best method to measure the strength of the plastic.

The plastic must be either molded or printed into a rectangular shape, called a dogbone, which is

placed inside of the mechanical analyzer. The dogbone is vibrated at varying frequencies over a

wide temperature range, and the analyzer measures the dynamic moduli of the plastic. The

vibrational frequency applied to the plastic is referred to as the stress, and the way the plastic

responds (displaces) is referred to as the strain. In ABS plastic, the stress and strain occur in

phase; the response in stress happens simultaneously with strain.4 The dynamic moduli being

measured are the storage modulus and the loss modulus. The storage modulus, G’, is the elastic

behavior of the material (the amount of energy it can store), and can be expressed as 𝐺! =

!!!!cos 𝛿, and the loss modulus, G’’, can be expressed as 𝐺!! = !!

!!sin 𝛿. The loss modulus is the

amount of energy the material loses as heat. The stress is represented as σ0, and the strain as ε0.

The 𝛿 represents the phase lag between stress and strain, and is the time it takes for the plastic to

strain in response to the applied stress. When the storage and loss moduli are plotted versus

                                                                                                               4 Yablon, Dalia G.; Gannepalli, Anil; Proksch, Roger; Killgore, Jason; Hurley, Donna C.; Grabowski, Jean; Tsou, Andy H. From Macromolecules (Washington, DC, United States) (2012), 45(10), 4363-4370. | Language: English, Database: CAPLU

temperature, the resulting plot gives structural information of the material, specifically the

transition from aligned polymer crystals, to a random polymer melt.

In order to determine if the CNT composite plastic is stronger, the analyses of the pure

ABS is compared to that of the CNT/ ABS. A group of researchers in Iran created eight ABS

dogbone test pieces5. The group prepares the two, four, and eight percent weight compositions of

CNT/ ABS plastic as mentioned above by using the two methods, direct mixing and solution

mixing.

First, the results of the solution mixing method will be compared to that of the pure ABS.

The plots below (Figure 4 and 5) are the dynamic mechanical analysis results obtained by the

group. The analysis was performed over a wide temperature and frequency range. Figure 4

corresponds to the solution mixing method, and figure 5 to the direct mixing method.

 

Figure 4: Storage and loss modulus of solution mixed ABS/CNT

                                                                                                               5 Mousavi, L., et al. (2012). "The effect of mixing process on linear viscoelastic and electrical properties of ABS/MWNT nanocomposites." Journal of Applied Polymer Science 125(S1): E260-E267.  

According to the plot, the pure ABS has the lowest storage and loss modulus,

corresponding to the least amount of strength. It appears that the strength of the plastic is

proportional to the amount of nanotubes that are mixed in. The 8% CNT plastic shows a

tremendous structural difference than that of the pure ABS.

Next, the results of the direct mixing are compared to that of the pure ABS. The results

the group reported can be found in the plot below.

 

Figure 5: Storage and loss modulus of direct mixed ABS/CNT

It is immediately apparent there is a difference between the strength of the plastic

depending on the method by which the CNTs are mixed. The storage modulus for the 8% CNT is

ten times less in the direct mixing method than that of the solution mixing method. For the

purposes of 3D printing, an ABS/ CNT composite that is produced by the solution mixing

method is the best way to provide greater structural strength. It is assumed the direct mixing

method did not mix the CNTs homogeneously, which would cause some parts of the plastic

dogbone to fail due to stress.

Now that the method to mix the CNT has been established to give the strongest plastic,

the analysis must be made on the architectural structure of the composite. To prove the CNTs

will generate the peg system to keep the layers stationary and bonded, electron micrograph

images were taken of an ABS/ CNT composite. In Figure 3, a stress has been applied between

the two plastic layers (top and bottom) and the carbon nanotubes are the ‘wires’ in between the

two layers.

 

 A perfect indication that the

CNTs will produce the peg

system is the fact that they are

connecting the two layers and

also embedded in each side.

When a new layer of plastic is

printed, the CNTs that are

embedded in the pliable stream of plastic

will embed within the interface where the

new stream of plastic heats up the old layer.

Figure 4 to the right shows the effect of too

much stress.

 

The  carbon  nanotubes  appear  relaxed  in  

Figure  4  because  they  are  no  longer  

connecting  the  two  layers.  A  large  enough  Figure 4: Micrograph image of ABS/CNT composite after nanotubes have broken between two layers.

Figure 3: Micrograph of ABS/CNT composite with applied stress.

stress  has  been  applied  that  the  nanotubes  have  snapped.  This  decreases  the  strength  

between  the  two  layers,  and  ultimately  decreases  the  overall  strength  of  the  entire  plastic  

piece.    

Conclusion  

The mechanical analysis performed on the two mixed composites shows that the method

by which the carbon nanotubes are mixed affects the strength of the final product. When an

object is being 3D printed, the filament is applied continuously, so the carbon nanotubes need to

be mixed as homogeneously as possible. From the plots, it is apparent that the solution mixing

method mixes the CNTs much more homogeneously and creates a stronger composite. It can be

seen in the micrograph images that the CNTs are fully embedded in the plastic layers. This may

not be the case of the direct mixing because each component is being mixed dryly without the

incorporation of a solvent.

This analysis is important for the expansion of 3D printing to larger scale applications.

Currently, a small car is the largest object that has been 3D printed. It was not printed all at one

time, but rather in segments that were later assembled. If more goods are produced by 3D

printing in the future, it is important to have an understanding of what causes variability in the

strength of the printed material. Overlooking the variables that affect the strength can cause

problems when trying to increase the scale of what’s being manufactured. If the car has printed

parts from different printers and research groups, the groups must understand that each part will

be subject to some stress, and therefore must decide which print method is the best choice. The

dry mixing method may prove to be much easier or cheaper, but the final filament product is

much different than if it had been prepared by solution mixing.

References

1. Sigma Aldrich Carbon Nanotubes- http://www.sigmaaldrich.com/technical- documents/articles/materials-science/single-double-multi-walled-carbon-nanotubes.html

2. Cumings, J.; Zettl, A. (2000). "Low-Friction Nanoscale Linear Bearing Realized from Multiwall Carbon Nanotubes". Science 289 (5479): 602–604.

3. Mari, D. and R. Schaller (2009). "Mechanical spectroscopy in carbon nanotube reinforced ABS." Mater. Sci. Eng., A A521-A522: 255-258.

4. Yablon, Dalia G.; Gannepalli, Anil; Proksch, Roger; Killgore, Jason; Hurley, Donna C.; Grabowski, Jean; Tsou, Andy H. From Macromolecules (Washington, DC, United States) (2012), 45(10), 4363-4370. | Language: English, Database: CAPLU

5. Mousavi, L., et al. (2012). "The effect of mixing process on linear viscoelastic and electrical properties of ABS/MWNT nanocomposites." Journal of Applied Polymer Science 125(S1): E260-E267.