Carbon Multiwall Nanotubes as a high performance conductive additive for demanding plastic applications

M

Res in Proper ty Re ten t ion – Comparitive Study

ultiwall carbon nanotubes were first synthesized in 1983 by scientists at Hyperion Catalysis International. These nanotubes are about 10 nanometers in diameter and 10

microns long. They are made by a continuous, catalyzed gas phase reaction of low molecular weight hydrocarbons. Current production capacity using this process is in the multiple tens of tons, with the capability to readily expand to meet demand. Figure 1 (Appendix) is a drawing of the graphitic multiwall structure, Figure 2 (Appendix) is a transmission electron microscope image of a portion of a nanotube showing the multiwall structure surrounding the hollow core, Figure 3 (Appendix) shows the curvilinear structure of multiwall nanotubes. Figure 4 shows the relative size of nanotubes compared to carbon fiber and carbon black.

Carbon nanotubes have proven to be an excellent additive to impart electrical conductivity in plastics. Their high aspect ratio (1000:1) imparts electrical conductivity at lower loadings compared to carbon black, chopped carbon fiber, or stainless steel fiber, see Figure 5. The benefits of lower loadings of conductive additives, on both polymer properties and polymer viscosity, will be discussed in the balance of this paper.

A study has recently been completed evaluating

three commercially available PC/ABS conductive compounds made with nanotubes, carbon fiber and carbon black. These three compounds were developed to offer approximately the same surface resistivity. Because of the different aspect ratios of the three additives, the level of nanotubes needed to obtain similar resistivities is much lower, see Table 1.

Nanotubes preserve more of the neat resin's elongation at break and unnotched Izod compared to carbon black or carbon fiber. The addition of any particulate additive to engineering resins results in a decrease in resin ductility. This can be dangerous in applications where loss of resin toughness can hurt the performance of a part. The small size and low loading of nanotubes minimizes the adverse

Nanotubes Maintain More of The Resin's Ductility

MR. PATRICK COLLINS, MARKETING MANAGER,HYPERION CATALYSIS

Carbon Nanotubes

NANO TECHNOLOGY

Carbon Fiber

Nanotubes

Nanotube Agglomerate

Carbon Black

Nanotube

Fig. 4. Comparison of nanotubes with carbon fiber and carbon black

Fig. 5. Calculated Loading for Percolation as a Function of Aspect Ratio

Loading

40%

35%

30%

25%

20%

15%

10%

5%

0%Spheres (L/D=1)

Carbon Black(L/D~10)

Carbon Fiber(L/D~100)

FIBRIL nanotubes(L/D~1000)

Theoretical Percolation Threshold

Table 1. Resistivity vs. Additive/Loading in PC/ABS

Additive

None

Nanotubes

Carbon black

Carbon fiber

Loading

wt. %

7.3

16.7

13.7

Volume Resistivity

(ohm-cm)

1016

101 - 103

<103

<103

Surface Resistivity

(ohms/sq)

n.a

104 - 106

<106

<106

effect on the ductility of the resin, see Table 2.

Because of their small size and low loading, nanotubes have less of an effect on part surface quality. The addition of most particulate additives to thermoplastics results in a decrease in the surface quality of the part which is detrimental when making appearance parts for automotive and for m a n y e l e c t r o n i c applications, as will be e xp l a i ned l a t e r. A numerical measure of surface smoothness was made using a Mahr Federal Perthometer on plaques molded in a mirror surface tool. Table 3 shows the arithmetic average of the surface roughness.

Nanotubes are very much smaller than other additives, thus are more insensitive to shear. The result is they form isotropic (random) distributions within molded parts. Large additives are frequently affected by the levels of shear commonly found in injection molding. This can give uneven distribution of the additive within a part, especially one that has corners, openings or other three dimensional details. For conductive additives this means uneven levels of conductivity at different spots on a molded part. Figure 6 (Appendix) shows a photomicrograph of a microtomed section of the carbon fiber filled injection molded tensile bar. At 2 3 0 x magnification it is easy to see the alignment of the carbon fibers in a section of the

Nanotubes give the smoothest part surface

Nanotubes give low part warpage

part. Figure 7 (Appendix) shows a Transmission Electron Microscope (TEM) view of an ultramicrotomed section of the nanotube-filled tensile bar. It can be seen that the nanotubes are randomly aligned. This insures a uniform level of conductivity throughout the most complex part or for large parts with multiple gates. Another advantage of the isotropic distribution of nanotubes is a reduced chance of part warpage. Table 4 shows the difference in shrinkage in the flow direction vs. shrinkage in the transverse direction for the three compounds.It can be seen that the differential shrinkage for the very high aspect ratio nanotube-filled compound is almost the same as the nearly spherical carbon black and much less than for carbon fiber. This means that part warpage will likely be much lower with nanotubes than carbon fiber.

Another advantage of the low loading of nantoubes is that they do not raise the viscosity of the compound as much as the higher loading of larger

fillers, see Figure 8. This means that thin walled or large multi-gated parts may be more easily filled.

Cleanliness of conductive plastic components is a vital consideration in clean-room manufacturing. Sloughing of particles as components slide against each other can contaminate delicate components. In addition, volatilization of organic compounds, such as sizing or coupling agents, can be a source of contamination. Fortunately, conductive compounds based on nanotubes are characterized by low sloughing and chemical cleanliness. Historically, carbon fiber has been used as a low-

Nanotubes give the least effect on resin viscosity

Resin Cleanliness – Comparitive Study

Table 2. Effect of Additive/Loading on Ductility

Additive

None

Nanotubes

Carbon black

Carbon fiber

Loading

wt. %

7.3

16.7

13.7

Elongation At Break

(%)

100

10+

3

1 - 3

Unnotched Izod

(ft lbs)

NB

30

10

4

Table 3. Average Surface Roughness (Ra) vs. Additive/Loading

Additive

None

Nanotubes

Carbon black

Carbon fiber

Loading

wt. %

7.3

16.7

13.7

Ra

(u m)

0.019

0.025

0.035

0.426

Table 4. Effect of Additive/Loading on Differential Shrinkage

Additive

None

Nanotubes

Carbon black

Carbon fiber

Loading

wt. %

7.3

16.7

13.7

Differential Shrinkage (a)

1.03

0.96

0.97

0.92

(a) Ratio of shrinkage in flow direction divided by shrinkage in transverse direction.

Fig. 8. Effect of Additive Loading on Resin Viscosity

Apparent Viscosity at 280C

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+00 1.E+01 1.E+02 1.E+03

Frequency (Rad/s)

Vis

co

sit

y(P

as

)

PC/ABS

PC/ABS + 7.3% NF

PC/ABS+ 13.7% CF

PC/ABS+ 16.7% CB

NANO TECHNOLOGY

sloughing conductive additive in polycarbonate (PC), which is itself considered to be a clean, low sloughing resin. To evaluate the relative cleanliness of compounds made with nanotubes, liquid particle count, abrasion resistance, and outgasing tests were conducted on commercially available grades of polycarbonate containing both carbon fiber and carbon nanotubes. The details of the studies follow.

One method for measuring surface cleanliness or resistance to sloughing is the Liquid Particle Count (LPC) test. Three different injection molded test plaques made from commercially available grades of polycarbonate – one neat, one containing 3% nanotubes, and one containing 10% carbon fiber – were tested (the loading differences reflect the concentration of nanotube and carbon fiber additives required to reach equivalent levels of conductivity.)In this test, a 50 x 90 x 3 mm plaque was immersed in 50 ml of deionized water and then subjected to

ultrasonic energy for 1 min at 45 kHz. In order to eliminate any differences in processing, handling, and exposure history, this first bath of water was exchanged for fresh deionized water and the plaque was again subjected to a cycle of ultrasonic energy at the same frequency for 1 min. Subsequently, 10 ml of the water were aspirated into a liquid particle counter. Three samples of the aspirated water were tested to reach an average particle count per square-centimeter of part surface area. Results are presented in Table 5 and

Liquid-particle Count

Figure 9. Data from this test indicate that there is significantly less contamination from samples containing nanotubes than with carbon fiber at both the 2-3 micron and > 3 micron particle size. The same data are presented graphically below.

Another test to determine suitability for use in a clean-room environment is abrasion resistance. Abrasion caused by two surfaces sliding together may lead to sloughing of particles that can contaminate clean-room components. Three commercial polycarbonate grades – neat PC; PC with 3% carbon nanotubes, and PC with 10%

carbon fiber – were evaluated in a reciprocating-motion wear test. A 1 x 50 mm glass plate under a 718 g load was moved across a molded plastic plate at a speed of 300 mm / sec for a total of 5,000 cycles. At the end of the test, the depth of the

abrasion on the plastic part was measured and the results are presented in the Table 6 and Figure10 below.

Improved Abrasion Resistance

Figure 10. Relative Abrasion Resistance

Neat PC PC + Nanotubes PC + Carbon Fiber

Test Sample

Depth

of P

art

Abra

ded

(mic

rons)

40

35

30

25

20

15

10

5

0

Susceptibility to AbrasionTable 5. Relative Sloughing Resistance

Sample Plaque

Neat PC (control)

PC with nanotubes

PC with carbon fiber

Filler Loading

(Wt-%)

0

3

10

Volume Resistivity

(Ohm-cm)

1x1014

3x106

1x107

Number of Particles (particles/cm2)Particle Size

2-3 micron

29.0

51.8

324.9

> 2-3 micron

29.0

51.8

324.9

Number of Particles per cm2 of Sample

2000

1500

1000

500

0Neat PC PC + Nanotubes PC + Carbon Fiber

Test Sample

Num

ber

of P

art

icle

s/2

cm

>3 microns

2.3 microns

Fig. 9. Relative Sloughing Resistance

Table 6. Relative Abrasion Resistance.

Sample Plaque

Neat PC

PC + nanotubes

PC + carbon fiber

Filler Loading

(Wt-%)

0

3

10

Volume Resistivity

(Ohm-cm)

1x1014

3x106

1x107

14

16

40

Depth of Surface

Scraped (microns)

Fig. 11. Relative Compound Outgasing

PC + Nanotubes

Test Sample

1.00E+05

8.00E+04

6.00E+04

4.00E+04

2.00E+04

0.00E+00

Relative Compound Outgasing

PC + Carbon Fiber

To

tal

Vo

lati

les

NANO TECHNOLOGY

Data from this test indicate that the depth of abrasion is significantly less from samples containing nanotubes than with carbon fiber. This means that there is a lower likelihood of sloughing in a sliding wear environment.

Since electronic components can experience thermal cycling during various production steps or in use, it is important to understand if any chemicals are likely to outgas during such operations, as they can damage delicate microelectronics. Two commercial compounds – polycarbonate with 3% nanotubes, and polycarbonate with 10% carbon fiber – were exposed to temperatures of 80C and 180C. Volatiles outgased were taken to be the total area under the gas chromatography peaks. Results are shown in Figure 11. Note: in this test, no control (neat polycarbonate) sample was tested because neat PC would have had fewer compounding steps (heat histories) than the compounds with conductive additives. Thus the baseline of volatiles would not be the same.The compounds with nanotubes showed substantially less outgasing than those with carbon fiber. This means that compounds made with nanotubes will have a lower risk of contaminating delicate electronics during heat cycling, either during part manufacture or in-use.

Carbon nanotube-filled plastics are being used in a wide range of commercial electronic applications in North America, Europe, Japan and the Pacific Rim. In the manufacture of semiconductor chips, the smallest static discharge can obliterate the small f ea t u r e s t ha t we r e so pa i n s t a k i ng l y photolithographically etched on the surface. During the manufacturing process, the wafers are transported from station to station in FOUPs (Front Opening Unified Pods) that have conductive plastic in all wafer contact points. But it is not enough for the plastic to be just ESD conductive, the part surface should be very hard and smooth to prevent sloughing of particles as the wafers are inserted and removed in the FOUP. In the hyperclean environment needed for chip manufacture, one free particle can destroy hundreds of thousands of dollars of product. The low sloughing characteristics of nanotube-filled plastics are thus vital to this industry.

Less Outgasing of Volatiles

Applications

Appendix

Fig. 1. Structure of Multiwall Carbon Nanotubes

The number of

shells varies. Eight

is typical.

Shells are rolled graphite

sheets

Approx.

5 nm

Approx. 10 nm

Copyright 2000

Copyright 2000

Fig. 2. Photomicrograph Showing Nanotube Wall Structure

Copyright 2000

Figure 3. Photomicrograph of Dispersed Nanotubes

Fig. 7. Transmission Electron Micrograph of Ultramicrotomed Section of Nanotube-filled Injection Molded Bar

Photos Courtesy of CRIF

Fig. 6. Light Transmission Photomicrograph of Microtomed Section of Carbon Fiber-filled Injection Molded Bar(Photo Courtesy of CRIF)

X230 IF

NANO TECHNOLOGY