Manufacturing Tough Amorphous Thermoplastic-Graphene Nanocomposites

Sudheer Bandla, Jay C. Hanan*

Mechanical and Aerospace Engineering, Oklahoma State University, Tulsa, OK- 74106

*Jay.Hanan@okstate.edu

ABSTRACT Poly(ethylene terephthalate) (PET)–graphene nano-

composites were prepared using high speed industrial scale injection molding. Nanocomposites with 0.6% and 1.2% of graphene platelets weight fractions were injection molded using a blended master-batch. X-ray tomography and ultrasonic imaging were applied for non-destructive analysis of the nanocomposites to evaluate the processing method. Mechanical testing of the nanocomposites show an improvement in stiffness up to 8% with a slight increase in the strength and toughness compared to the control. The matrix of the nanocomposites prepared was amorphous. Furthermore, increase in the stiffness was corroborated based on numerical predictions.

Keywords: Nanocomposites, Toughness, Thermoplastic, PET, Injection molding.

1 INTRODUCTION Polymer nanocomposites have been studied for more

than two decades because of their superior performance in comparison with traditional composites [1]. Depending on the type and extent of the nanoparticles used, polymers can be engineered for a range of applications including structural, packaging, and electronic. The potential for performance enhancement is substantial, even with reinforcement phase fractions less than 1% [2]. However, few products made using nanocomposites are available commercially. In contrast, the extensive use of micro-composites is due to their ease in manufacturing and the low cost of reinforcements. In the case of nano-reinforcements, achieving uniform dispersion in the polymer matrix, unknowns in manufacturing, and characterization costs remain significant constraints [3]. A defect free process with industrial scalability is critical for their commercial success.

With this objective, Polyethylene Terephthalate (PET)-

Graphene nanocomposites were injection molded using a master-batch approach [4]. In our previous work [5], nanocomposites with graphene loading fractions as high as 15% were prepared using a two stage mixing process. They exhibited up to a 3 fold increase in elastic modulus. However, a significant drop in strength and toughness of the nanocomposite was observed over the base polymer [6].

In the current work, a new set of tough and amorphous nanocomposites were prepared by applying a similar approach through an improved process. Comparisons were made between the two sets of nanocomposites to highlight the differences from processing and their influence on the final mechanical behavior.

1.1 Polyethylene Terephthalate – Graphene

Nanocomposites

Thermoplastic polymers provide greater flexibility on the molding processes compared to thermosetting resins. There are also advantages in recyclability. Polyethylene terephthalate (PET), an aromatic semi-crystalline thermoplastic polyester was selected as the matrix material. Synthesized in the early 1940’s, it is well known for its physical strength, optical clarity, and chemical resistance. PET finds its largest volume applications in fiber, packaging, filtration, and thermoforming industries. Linear PET is naturally semi-crystalline. Heat treatment and mechanical history influences the amorphous content [7]. PET nanocomposites can be useful for different applications. Several works of PET-nanocomposites have been reported, with different types of nanoparticles like Clay, Carbon (SWNT’s [8], MWNT’s [9] and graphene [5, 10, 11]) and oxides.

Graphene is considered superior over other carbon

nanoreinforcements such as CNT’s, CNF’s, and Expanded Graphite (EG), in terms of its aspect ratio, flexibility, transparency, thermal conductivity and low CTE [1]. It is this superior nature and cost comparison with carbon nanotubes that makes graphene a better nano-scale reinforcement for nanocomposites with multiple property enhancements [1, 12]. PET-graphene nanocomposites are less well understood compared with other reinforcements.

1.2 Manufacturing Processes

The value of adding nanoreinforcements in polymers is better appreciated by means of an optimum scalable manufacturing technique. Traditional composite manufacturing processes are not necessarily efficient or effective for making nanocomposites. Achieving complete dispersion of graphene in a polymer matrix is difficult as they tend to agglomerate due to high surface energy.

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Surface treating is one option to reduce agglomeration, but can increase the processing cost.

Several processes like Melt Compounding, Solid-State

Shear Pulverization (SSSP), Layer-by-Layer (LBL) manufacturing, in-situ polymerization, and Master-batch processing were introduced of which only a few were scalable. Melt-compounding and in-situ polymerization are the most used for the processing of PET nanocomposites. However, in-situ polymerization may not be effective at an industrial scale because of cost, and the process involves numerous variables. Whereas, melt-compounding is scalable for large scale processing of nanocomposites. In this approach, the nanoreinforcements disperse into the polymer matrix by means of mechanical shearing.

2 MATERIALS AND METHODS Polyethylene terephthalate of 0.80 dl/g I.V. grade

commercially known as oZpet™ (supplied by Leading Synthetics, Australia) was used along with exfoliated graphene nanoplatelets (xGnP-M-5, supplied by XG Sciences, Inc.) for the preparation of nanocomposites. As received nanoplatelets, shown in Figure 1(a) were compounded with PET, as described previously [5]. The master-batch pellets were loaded with graphene at 10% wt fraction.

2.1 Processing of Nanocomposites

PET-nanocomposites of the required weight fraction were molded using a Husky HyPET 90 ton, 38 mm screw injection molding system. The injection molder, equipped with water cooling is capable of running at low cycle times. The fast quench maintains an amorphous matrix in the composite. PET-graphene master-batch pellets were dired separately and fed into the system near the feed throat using a TrueFeed™ gravimetric feeder (ConAir Inc.). The final weight fractions of the nanocomposite samples were verified by measuring the pellet fractions in images collected from the feed throat, as shown in Figure 1(b). Nanocomposites of two different (0.6% and 1.2%) weight fractions were prepared using the master-batch approach. A customized specimen shape (tubular) was used for ease with processing. Samples obtained for the study were collected only after the process was stabilized.

Figure 1: (a) Graphene nanoplatelets, (b) Mixture of PET and

PET-Graphene master batch pellets at the feed throat.

3 CHARACTERIZATION STUDIES Nanocomposites collected were tested for their

mechanical performance using a custom fixture. They were analyzed non-destructively, using X-ray tomography and ultrasonic imaging techniques for comparison with previous studies [6]. Density measurements (based on the Archimedes principle) of the nanocomposite samples were collected to validate their amorphous microstructure.

3.1 Mechanical Testing

Tensile testing to failure of the nanocomposites was performed on a universal tensile tester (Instron 5582) with a 5 mm/min extension rate. A typical stress-strain curve of the nanocomposite sample is presented in Figure 2. Five nanocomposite samples of each kind were tested. The range of mechanical properties from these is indicated by the error bars in Figure 5.

Figure 2: Stress-strain curves of the nanocomposites

compared with control PET. Failure is > 40% strain for the

composites (Inset – curve for PET+2% Graphene

nanocomposite [6], note the failure at less than 2% strain).

3.2 X-Ray Tomography

X-ray microtomography was used for imaging nanocomposite samples to detect voids. Electron micrographs of the fracture surface from earlier work [6] showed voids within those nanocomposites. Tomography allows visualization of defects non-destructively. Radiographs of the nanocomposites, shown in Figure 3(a) and (b) were collected at 10 µm and 3 µm pixel resolutions respectively. Nanocomposites prepared through the improved process do not exhibit visible defects.

Figure 3: Radiographs of the nanocomposites: (a) 1.2% wt

nanocomposite without voids and (b) a 10% wt nanocomposite

(from [6]) showing voids.

100 µm

(b)(a)

0

10

20

30

40

50

60

0 1 2 3 4 5

Str

ess

(MP

a)

Strain (mm/mm)

PET

PET+ 0.6%

PET+ 1.2%

0

10

20

30

40

50

0 0.01 0.02

Str

ess

(M

Pa

)PET+ 2%

(a)

1.5 mm

(b)

1.5 mm

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3.3 Ultrasonic Imaging

Ultrasonic imaging is another useful technique for imaging defects present in the samples. It provides better flexibility on the sample size, but is more limited on the shape (difficult to image samples with high curvature or surface roughness) compared with X-ray tomography. The ultrasonic micrograph shown in Figure 4 is a bulk scan for 5% graphene nanocomposites, scanned at a scan frequency of 30 MHz. It is evident from this micrograph that the voids are caused by processing.

Figure 4: Ultrasonic scan showing the presence of voids in

PET-5% graphene nanocomposites (arrow indicates direction

of mold fill).

4 RESULTS & DISCUSSION Using the stress-strain data from the tensile tests,

Young’s modulus, tensile strength, and toughness of the nanocomposites was obtained. The PET-graphene nanocomposites show a 4% and 8% increase in elastic modulus compared with the control sample. Samples with similar processing conditions and amorphous microstructure provide a way to study the role of nanoplatelets in improving the properties. Toughness was determined based on the area under the stress-strain curves. Based on a standard deviation of data from five samples, error bars were plotted, as shown in Figure 5. A small increase in strength was observed at higher loadings of graphene.

Nanocomposites prepared in the current work exhibit

superior toughness and strength in comparison with the values reported earlier [6]. Improvements in the process by moving from a lab scale injection molding unit to an industrial scale unit permitted faster cooling rates and injection speed. This allowed manufacturing void free and amorphous samples. Figure 2 compares the stress-strain curves of the current samples with that of the previously tested samples. Based on the evidence of voids from radiography, processing defects contributed to the decrease in strength of the nanocomposites. The weaker samples also had a higher crystalline phase fraction. The increase in the Young’s modulus was found to match the numerical modeling results in previous work [5].

Figure 5: (a) Young’s modulus, (b) tensile strength, and (c)

toughness of the nanocomposites compared with PET.

In general, an increase in stiffness of a nanocomposite is coupled with a decrease in properties like strength or toughness [13]. Here this was not observed. Processing of the nanocomposite clearly plays an important role in maintaining or improving toughness. An improvement in toughness by the addition of nanoreinforcements is often explained based by a crack bridging mechanism. Gersappe [14] found that the increase in toughness is also related to the mobility of the nanofiller and other factors like the level of interaction between matrix and filler or size of the filler. Further, Gersappe found that this is more effective at temperatures above the Tg of the matrix. Here the stiffness was shown to increase while the strength and toughness remained the same or increased. This could be related to the amorphous matrix of the nanocomposites, which permits the mobility of polymer chains and the nanoplatelets under stress. Further investigation is required on platelet distribution and orientation before and during uni-axial loading to understand the mechanism effectively.

5 CONCLUSIONS Polyethyelene terephthalate-graphene nanocomposites

with 0.6% and 1.2% loading fractions were processed

20 mm

2.0

2.2

2.4

2.6

2.8

3.0

PET PET + 0.6% PET + 1.2%

Mo

du

lus

(GP

a)

49

50

51

52

53

54

55

56

PET PET + 0.6% PET + 1.2%

T. S

tren

gth

(M

Pa)

100

110

120

130

140

150

160

170

PET PET + 0.6% PET + 1.2%

Tou

gh

nes

s (M

J/m

2)

(a)

(b)

(c)

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without void defects. The stiffness of the matrix was increased by 8% while retaining the strength and toughness of the matrix. Master-batch blending and injection molding can be implemented for processing all regular thermoplastic polymers and is applicable to large scale manufacturing. Further opportunities in terms of improving the properties are under investigation.

ACKNOWLEDGEMENTS The authors would like to acknowledge XG Sciences for

providing graphene nanoplatelets and Oviation Polymers for assistance with the compounding process. Further, we would like to thank Michelle Forbes, Sonoscan Inc., for ultrasonic imaging. Reaj Ahmed of Niagara Bottling LLC and Richard Plunkett of Husky Injection Molding Systems Ltd. aided in processing the samples. This work was possible through industry partnership with Oklahoma State University.

REFERENCES

[1] T. Kuilla, S. Bhadra, D. Yao, and N. Kim, "Recent advances in graphene based polymer composites," Progress in Polymer Science, vol. 35, pp. 1350-1375, 2010.

[2] M. A. Rafiee, J. Rafiee, Z. Wang, H. Song, Z.-Z. Yu, and N. Koratkar, "Enhanced Mechanical Properties of Nanocomposites at Low Graphene Content," ACS Nano, vol. 3, pp. 3884-3890, 2009.

[3] M. R. Loos and K. Schulte, "Is It Worth the Effort to Reinforce Polymers With Carbon Nanotubes?," Macromolecular Theory and Simulations, vol. 20, pp. 350-362, 2011.

[4] J. C. Hanan, U.S. Provisional Patent Application 61/482,048, "Polyethylene Terephthalate-Graphene Nanocomposites," 2011.

[5] S. Bandla and J. Hanan, "Microstructure and elastic tensile behavior of polyethylene terephthalate-exfoliated graphene nanocomposites," Journal of Materials Science,

vol. 47, pp. 876-882, 2012. [6] S. Bandla and J. C. Hanan, "Morphology and

Failure Behavior of Polyethylene Terephthalate-Exfoliated Graphene Nanocomposites," in 69th

Annual Technical Conference & Exhibition, Boston, 2011, p. 673.

[7] S. K. Sharma and A. Misra, "The effect of stretching conditions on properties of amorphous polyethylene terephthalate film," Journal of

Applied Polymer Science, vol. 34, pp. 2231-2247, 1987.

[8] A. Anand K, U. S. Agarwal, and R. Joseph, "Carbon nanotubes-reinforced PET nanocomposite by melt-compounding," Journal of Applied

Polymer Science, vol. 104, pp. 3090-3095, 2007.

[9] J. Y. Kim, H. S. Park, and S. H. Kim, "Multiwall-carbon-nanotube-reinforced poly(ethylene terephthalate) nanocomposites by melt compounding," Journal of Applied Polymer

Science, vol. 103, pp. 1450-1457, 2007. [10] H.-B. Zhang, W.-G. Zheng, Q. Yan, Y. Yang, J.-

W. Wang, Z.-H. Lu, G.-Y. Ji, and Z.-Z. Yu, "Electrically conductive polyethylene terephthalate/graphene nanocomposites prepared by melt compounding," Polymer, vol. 51, pp. 1191-1196, 2010.

[11] M. Li and Y. G. Jeong, "Poly(ethylene terephthalate)/exfoliated graphite nanocomposites with improved thermal stability, mechanical and electrical properties," Composites Part A: Applied

Science and Manufacturing, vol. 42, pp. 560-566, 2011.

[12] P. Mukhopadhyay and R. K. Gupta. (2011) Trends and Frontiers in Graphene-Based Polymer Nanocomposites. Plastics Engineering.

[13] D. Shah, P. Maiti, D. D. Jiang, C. A. Batt, and E. P. Giannelis, "Effect of Nanoparticle Mobility on Toughness of Polymer Nanocomposites," Advanced Materials, vol. 17, pp. 525-528, 2005.

[14] D. Gersappe, "Molecular Mechanisms of Failure in Polymer Nanocomposites," Physical Review

Letters, vol. 89, p. 058301, 2002.

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