CONTINUOUS PRODUCTION OF POLYCARBONATE

CONTINUOUS PRODUCTION OF POLYCARBONATECARBON NANOTUBE COMPOSITES

J.A. Covas l

(,)

, C.A. Bernardo 1 , O.S. Carneiro', J.M. Maia 1 ,

F.W.J, van Hattum', A. Gaspar-Cunha 1 , L.P. Biro 2 , Z.E. Horvath 2 , I. Kiricsi 3 , Z. K o n y a \ K. N i e s z 1 ]

IPC - Institute for Polymers and Composites, Department of Polymer Engineering, University of Minho, 4800-058 Guimaräes, Portugal 2 Nanostructures Laboratory, Research Institute for Technical Physics and Materials Science, Hungarian Academy of Sciences, H-l 121 Budapest, Hungary 1 Department of Applied and Environmental Chemistry, University of Szeged, H-6720 Szeged, Rerrich Bela ter 1, Hungary

ABSTRACT One of the main obstacles in current research in polymer carbon nanotube composites is the high price of carbon nanotubes and thus their availability in large quantities. This critically limits the range of techniques that can be used to prepare the composites. As such, the development of methods for their continuous production in small quantities can bring great benefit to the research in this area. The present text describes an investigation on the continuous, laboratoryscale, production of polymer/carbon nanotube composites by extrusion. The objective of the study was not only to make new materials, but also, through appropriate design, to enable the scaling-up of the production technique. Multi-wall carbon nanotubes (MWNTs) were made by chemical vapour deposition of acetylene on alumina supported transition metal catalysts. Composites were then produced in a purpose built micro-extruder and characterized at the nano- and macroscopic scales. (,)

Corresponding author:

Tel: +351 253 510320; Fax: +351 253 510339 e-mail: [email protected]

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Vol. 25, No. 1. 2005

Continuous Production of PolycarbonateCarbon Nanotube Composites

It was found that important processing parameters of the current microextruder were comparable to those of industrial extruders. Thus, it can be anticipated

that the quality

of composites produced

with

it will

be

comparable to that of composites of the same system produced in large-scale equipments. Keywords:

carbon

nanotubes;

polymer

composites;

rheology;

melt

processing; extrusion; scale-up

1 INTRODUCTION The discovery of carbon nanotubes (CNTs) more than a decade ago by Ijima /1 / marked the opening of a new chapter in nanoscale materials science. Active research followed that discovery 121. After all these years it is becoming evident that, in order to keep the research

momentum,

a

breakthrough on applications is necessary. At present, a major limiting factor to the success of any application is the relatively high cost of nanotubes, which results from low reaction yields and low activity. If the production costs can be cut down, and the availability problem is solved, field emission devices may probably be the first large-scale application to reach the market. Another interesting application, resulting from the high stiffness, flexibility and strength of CNTs (their tensile strengths can be 7 to 10 times higher than that of high quality steel), is their use as matrix reinforcements in composites 12)1. Polymers, namely thermoplastics, seem ideal candidates for matrices, as they have good processability and versatility, in spite of generally low mechanical properties. Further to the reinforcing action, providing electrical and/or thermal conductivity to thermoplastic matrices may be an added advantage of CNTs. However, there are large problems still to be overcome in this application, namely poor adhesion to the matrix and a strong tendency to bundle up, with the consequent difficulty in achieving a good dispersion. Research efforts on the use of CNTs as thermoplastics' reinforcements, should then focus on mastering the compounding and processing know-how. In spite of this, to the authors' knowledge, only a limited number of scientific publications dealt with CNT-thermoplastic composites /4/, and even fewer involved melt processing, the technologically relevant method to make composites 15-91. Furthermore, except for the work of Pötschke et al. /e.g. 8, 9/, none of these studies used conditions similar to those prevailing in

40

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Journal of Polymer

J. A. Covas et al.

Engineering

industrial compounding operations. This is not surprising, considering that it is usually necessary to have material quantities in the order of kilograms for proper thermoplastics composite's processing research and the high cost of CNTs. Moreover, as Pötschke's work involved a commercially available CNT/polymer masterbatch /10/, it circumvents the problems associated with the initial distribution and dispersion of CNTs in the polymer during compounding and does not involve direct characterization of the nanotubes, as dealt with in this paper. This significantly limits the comparability of Pötschke's results with those obtained with other CNT/polymer composites, commonly non-predispersed. In conclusion, the availability of well-characterized carbon nanotubes is again the rate limiting factor of CNT-composite research. But the situation can be solved in a different way. For example, if it is possible to develop a technique to produce

small

amounts of polymer/CNT

composites,

in

conditions similar to those prevailing in industry. This is exactly the aim of the present investigation. The work involved the development and use of a small continuous melt mixing/extrusion device. This apparatus is capable of promoting good levels of distributive mixing, using gram amounts material

and

making

samples

adequate

for

rheological

and

of

tensile

characterization tests. The samples were also analyzed by Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). Integration of experimental information from both the nano- and macroscales with processing data, allowed the evaluation of the feasibility of scaling-up the production technique. With this knowledge it may be possible to establish an industrially viable procedure to make high-performance composites.

2 MATERIALS AND EXPERIMENTAL PROCEDURE 2.1 M a t e r i a l s In the present work, a polycarbonate injection moulding grade (Bayer Makro Ion 2805), with a MFI of 7-10 (DIN 53735, 300/1.2), was used as the matrix material. This polymer was chosen

because it combines

good

engineering properties with moderate price. The work of Pötschke et al. /8/ already showed the beneficial degree of wetting and phase adhesion of carbon nanotubes in this matrix system. Additionally, some of the authors had previous experience in using polycarbonate to prepare composites with similar entities, vapour-grown carbon nanofibres (VGCFs), and study their

41



Vol. 25, No. 1, 2005

mechanical and rheological properties / I 1 , 12/. The outcome of this previous research

showed

the

feasibility

of

producing

these

composites

with

commercial polymer processing equipment. Furthermore, besides enhancing transport properties, the incorporation of VGCFs had a reinforcing effect on the matrix /13/. In the present study, the small dimensions of the mixing equipment made it necessary for the polymer to have a corresponding small particle size. Hence, prior to the compounding, the polymer pellets were immersed in liquid nitrogen and immediately micronised in a Retsch Ultra Centrifugal M i l l Z M 100 with a 1mm filter mesh.

2.2. Production and characterization of the nanotubes Carbon nanotubes were produced in the authors' laboratories by chemical vapour deposition catalysts.

(CCVD) of acetylene on alumina

After the reaction

the

material

supported

was chemically

cobalt

purified

in

hydrochloric acid in order to remove traces of catalyst. Further details of the production and purification techniques can be found elsewhere /14/. The main

advantage

functionalized

of

this

nanotubes,

technique in

small

is

the

amounts.

possibility This

of

is very

producing useful

for

preliminary studies such as that described herein. The carbon nanotubes were ball milled prior to the production of the composites in order to facilitate their dispersion in the polymer matrix by decreasing the degree of entanglement. It is well known that C C V D produced nanotubes usually show a mild random curvature and their length may reach several tens of micra. It was shown that by using a ball milling procedure (a special heatable metal mortar with several small metal balls /15/) the length of CNTs can be reduced in a controlled way, i.e., the mechanical cut can result in objects with

100-300 nm in length. When ball milling

was

performed in reactive atmospheres, such as NH 3 or COCI 2 , functionalized short nanotubes were obtained with functional groups attached such as NH 2 /-CONH 2 and -COC1, respectively. A detailed description of the ballmilling process as well as the CNT-characterisation by X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (IR) can be found in /16,17/. In this study the carbon nanotubes were examined by TEM (Philips C M 2 0 Twin microscope), as purified and after ball milling. For that, ultrasonicated suspensions of C N T s in ethanol were drop-dried on carbon film coated slot grids.

42


J.A. Covas et al.

Journal of Polymer

Engineering

2.3 Preparation of the composites T h e c o m p o s i t e s w e r e p r e p a r e d in the micro-extrusion line illustrated in Figure 1. T h e system c o m p r i s e s a vertical single screw extruder (described in detail elsewhere /18/), a 4 m m diameter circular die, a w a t e r c o o l i n g bath and a pulling device and is able to operate at very low t h r o u g h p u t s ( f r o m 6 to 3 0 g r a m s per hour). A s in industrial practice, it provides c o n t i n u o u s extrusion o f material in a single-pass a l o n g the screw, under controlled t e m p e r a t u r e and speed.

Fig. 1:

Micro extrusion

line. Left: General

layout; right: single

screw

extruder.

T h e extruder w a s designed in order to induce t h e r m o - m e c h a n i c a l histories similar to those found in normal industrial lines. Available scale-up strategies were initially a d o p t e d . T h e s e rules impose either constant global conditions (e.g., shear rate), or analyse separately each main process p a r a m e t e r (e.g., solids c o n v e y i n g rate, melting rate, p u m p i n g rate, residence time, total shear strain, p o w e r c o n s u m p t i o n and ratio of barrel surface area to t h r o u g h p u t ) . Unfortunately, practice s h o w e d that their application yielded g e o m e t r i e s that were dimensionally incoherent. This is probably due to the fact that not only heat conduction b e c o m e s much more efficient for small thicknesses (the time for heat penetration

is approximately

proportional to the square of the

distance), but also as the barrel diameter decreases, the barrel s u r f a c e area to volume

ratio increases, so that more external

Therefore,

higher

heat

transfer

efficiency

heat can be

than

anticipated

transmitted. and

less

contribution f r o m viscous dissipation is e x p e c t e d .

43


Vol. 25, No. 1. 2005


Given the above, screw design was assisted by a plasticating extrusion modelling package developed by the authors /19/. The routine considers the plasticating

process

from

hopper to die, by

including

the

sequential

contribution of gravity solids flow in the hopper, drag solids conveying, melting, melt conveying in the screw and die flow. Figure 2 shows the predicted axial melting (in terms of the usual solids, X , to channel, W , width ratio), melt pressure and shear rate profiles for one of the existing microscrews. Under the operating conditions indicated, the system should yield circa 24 gram/hour. Melting is almost instantaneous, due to the very efficient heat transfer between the barrel and the very thin solid bed (as the barrel diameter decreases, the barrel surface area to volume ratio increases, so that more external heat can be transmitted and less contribution from viscous dissipation is expected). In practice, upon operation, the system reaches steady state very quickly, and hence it is possible to produce samples from very little amounts of material. Several screw profiles are available, with and without mixing sections, and with different compression ratios. In all cases, special constructive and operational techniques were developed, in order to overcome the solids conveying difficulties of the polymer powder /18/. Preliminary

extrusion

experiments

were

performed

using

neat

polycarbonate (PC), in order to establish an adequate operating window (hopper cooling water flow rate, barrel feed section cooling air flow rate, barrel and die temperature, screw speed, haul-off rate). Then, PC and the corresponding amount of C N T s were tumble mixed, the mixture being fed to the

extruder

hopper.

However,

despite

this, segregation

of the

two

components was evident, due to the agglomeration of the nanotubes caused by van der Waals interactions. With barrel and die set temperatures of 280°C, screw speed of 40 rpm and haul-off roll rotating at 18 rpm, 5 gram extrudates with circa 1mm in diameter were produced for each C N T s ' concentration (0.25, 0.50, 0.75, 1.00, 1.50, 2.00 and 3.00 % w/w).

2.4 Morphological characterisation of the composites Composite specimens were fractured after immersion in liquid nitrogen. The resulting fracture surfaces were gold sputtered and observed by S E M (Cambridge Stereoscan 360). Several magnifications (from 400 to 12000 x) were used, to assess both the qualitative dispersion of the nanotubes in the matrix and the capability of the polymer melt to wet the nanotubes and/or to penetrate their agglomerates.


Journal of Polymer Engineering

J.A. Covas et al.

HI = I.I mm

4

H3 = 0.4 mm

J

Tb = 300 °C

N - 60 rpm

L2 = 22 mm

L3 = 22 mm

II

LI =20 mm

L = 64 mm

1.0

100.0 X/W — — — Shear rate

0.8

80 0

Pressure ^

OL &

0.6

60 0

0.4

- 40.0

0.2

• 20.0

J= (/)

s. υ 3 4> £

4- 0 0

0.0

0.00

Fig. 2:

;>

0.01

0.02

0.03

0.04 Um)

0.05

0.06

Predicted axial melting (in terms of the relative channel,

0.07

0.08

solids,

X, to

W, widths), melt pressure and shear rate profiles for one of

the available micro-screws. The screw geometry and the operating conditions are also indicated. The composites were also examined by TEM. For this, samples were ultramicrotomed from disks similar to those utilized in the rheological experiments (see 2.5), with a diamond knife with the edge always parallel to the disk axis. Sections with a thickness of about 50 μηι were cut and transferred onto carbon foil coated slot grids. The samples were then investigated in the TEM equipment using a 200 kV acceleration voltage.

2.5 Rheological and mechanical characterisation of the composites The samples for rheological testing were prepared in two steps. First, the extruded composite filaments were milled in a Retsch Ultra Centrifugal Mill ZM 100 high-speed, liquid nitrogen-cooled mill. Afterwards, the resulting powder was molten and compressed into small disks, which were then tested. Three types of tests were carried out on the disks, at a temperature of 280 °C, in a Reologica StressTech HR controlled-stress rotational rheometer, using a 25 mm parallel-plate geometry, with a gap of 700±2 μιτι:


Vol. 25, No. I, 2005


a) Stress sweeps, from 0.12 to 12000 Pa, at fixed frequencies of 0.1, I and 10 Hz, to assess the range of linear viscoelastic behaviour of the materials; b) Frequency sweeps, from 0.03 to 30 Hz, in a stress range corresponding to linear viscoelastic behaviour; c) Shear stress sweeps in steady shear, upwards from 0.2 Pa until the onset of flow instabilities. Composite filaments were tensile tested in a Minimat miniature material tester at a gauge length of 20 mm and a test speed of 2 mm/min. For comparison, neat polycarbonate samples were also tested. Prior to testing, the filament diameter was measured at several positions along its length. The force-displacement data obtained were used to calculate Y o u n g ' s modulus and strength of each filament. For each nanotube concentration, the data on at least five samples were used to calculate the average properties.

3 RESULTS AND DISCUSSION 3.1 Characterization of the nanotubes As mentioned earlier, the carbon nanotubes were investigated by T E M with and without ball milling. After purification, the samples contained approximately 9 5 % of multiwall carbon nanotubes, with a very small fraction of catalyst support material and graphite coated metallic particles. Typical micrographs are shown in Figure 3(a) and (b). As can be observed in Figure 3, the outer and inner diameters of the nanotubes were typically in the 5-30 nm and 3-5 nm ranges, respectively. The T E M images also show that, after ball milling, the lengths of the nanotubes, originally in the order of the tens of micra, were reduced to an average value of 1 to 3 microns. The ball milled nanotubes were found to exist partly in aggregates of a few micra size, which were held together only by weak van der Waals forces and tended to bundle-up with time and upon solids' flow, eventually reaching dimensions up to 30 micra.

46


J. A. Covas et al.

Journal of Polymer

Engineering

Fig. 3: TEM images of the as purified (a) and ball milled (b) CNTs. The larger number of nanotube ends in the ball milled sample is due to their smaller average length.

3.2 Properties of the composites SEM observations of the composites evidenced a good distribution of the CNTs in the polymer matrix. Separate, well dispersed nanotubes appear in the matrix, as can be seen in the micrograph in Figure 4(a). However, some of them are also arranged in agglomerates with average diameters ranging typically from 1 to 10 μιη (see Figure 4(b)). Slightly bigger agglomerates, for higher nanotube concentrations, were also detected. This is in agreement with findings from Bai et al. 1201, where nanotube agglomerate size increases with nanotube concentration. In their work, which used manual homogenisation in an epoxy matrix, agglomerates with sizes of up to several tens of micra are found /see 20, Fig. 5/. This emphasises the advantages of higher shear meltmixing processing methods, such as that used in the present work. It can thus be concluded that the micro-extruder was able to diminish the dimensions of these initial agglomerates, but not to totally break them. Good polymer penetration, even in the largest of the agglomerates (such as that shown in Figure 4(c)), was evident. In general, the TEM investigation supported the SEM observations. Figure 5(a) shows individual nanotubes surrounded by matrix material. Again, the density of individual CNTs was found to be proportional to the composites'

nanotube

content, as well

as the appearance

of

some



Vol. 25, No. 1, 2005

agglomerates

(Figure

5(b)). The

penetration

of the polymer

into the

agglomerates seemed to be relatively good (Figure 5(c)). Interestingly, a few voids

could

be

observed

in the

inner,

most

compact

areas

of

the

agglomerates, as shown in Figure 5(c) (the voids are indicated by arrows). Similar voids seem to be present in the nanotube PC composites studied by Pötschke et al. /21/.

(c) Fig. 4:

SEM micrographs of the PC/1.5% C N T composite: (a) area with good CNTs dispersion (4000x); (b) section with agglomerates of different dimensions (4000x); (c) partial view of a 13x25 μηι agglomerate (12000x).

It is worth pointing out that during sample preparation for TEM, when the diamond knife was cutting slices from the disks, a high shear was applied. However, in spite of this, CNTs pull out was never observed in any of the samples. It can therefore be concluded that the adhesion between the matrix and CNTs, at the macroscopic scale, was high enough to prevent separation during cutting, which is coherent with the increased CNT-matrix adhesion as


Journal of Polymer Engineering

J.A. Covas et al.

observed by Pötschke et al. in a similar system /8/. Also, in contrast with samples prepared by the solution-evaporation

method 1221, no polymer

defects caused by shrinkage could be found. * , •

S I» ^

J' er1

•

·

* ' *r

r

J

·|>·

^W'ivA" o> > >»,

«

is ,

%

Wt» ..« «fee . *. ·

?

.«*

Us -ψ '

r

m. \

500 nm (a)

Fig. 5:

(b)

Low magnification T E M images of C N T composites: (a) individual nanotubes and (b) compact agglomerate in the PC/0.75% C N T composite; (c) partial view of and agglomerate in the PC/ 1%CNT composite: matrix free voids indicated by arrows. The slant stripes result from cutting knife imperfections.

The rheological data in Figure 6 corresponds to the result of the steady shear tests. From the figure it is quite clear that the addition of carbon nanotubes to the polycarbonate has several effects. Firstly, there is a very strong reinforcement effect on the viscosity. For example, it increases by approximately four orders of magnitude when 3 w/w % nanotubes are added. In fact, for any given C N T concentration, three zones of different flow behaviour can be clearly distinguished: i) at low stresses there is a very large increase in viscosity, a behaviour that is comparable to that of liquids presenting a yield stress; ii) at intermediate stresses, there is a Newtonian


Vol. 25, No. I, 2005


plateau, in line with that of neat PC, but, as would be expected, displaced to higher absolute viscosity levels ( f o r example, it increases by approximately four orders of magnitude when 3 % w/w nanotubes are added); iii) at high stresses,

a

previously

non-existing

strong

shear-thinning

behaviour

is

observed, that eventually leads to viscosities lower than that of neat PC.

1.0E+08 1.0E+07 1.0E+06

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PC PC+0.5% PC+1.0% PC+1.5% PC+2.0% PC+3.0%

NT NT NT NT NT

****** % x

*x

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