Innovative ESD Thermoplastic Composites Structured
Through Melt Flow Processing
Moshe Narkis,
Department of Chemical Engineering, Technion - Institute of Technology, Haifa, Israel
Gershon Lidor, Anita Vaxman and Limor Zuri,
CarmelStat, Israel, POB 1468, Haifa, Tel: 972-4-8466016, Fax: 972-4-8646958, e-mail: [email protected]
Abstract
The amount of carbon black required to impart electrical conductivity to an insulating polymer can be
dramatically reduced by its selective localization in a multi-component system which includes the insulating
polymer. The present report describes property - structure relationships of polypropylene/nylon/glass fiber
(PP/PA/GF) composites with consistent resistivity levels within the 106 - 109 ohms/sq range achieved at very
low carbon black loadings (less than 2 %). The quaternary composites studied structure spontaneously during
the hot compounding/processing steps and have unique triple-percolation structures. The results were compared
with typical carbon black filled materials, which usually contain 15 to 20 wt% carbon black and are too
conductive to meet the 106 - 109 ohms/sq range.
Introduction
A variety of materials is available to package
sensitive electronic devices and prevent damage from
electrostatic discharge (ESD) during manufacturing,
assembly, storage and shipping. Polymer/carbon black
compounds are of special interest in numerous
applications in the electronic industry, because they
combine the good physical properties of commercial
polymers with the relatively high conductivity and
low cost of carbon black (CB), thus offering
interesting materials of commercial value.
Polymers, well known as good insulators, can become
conductors by their modification with a conductive
filler, e.g. carbon black, carbon fiber, metallic powder
or fiber, and glass spheres or glass fibers coated with
metals. A critical amount of a conductive filler,
known as the percolation threshold, is necessary to
build up a continuous conductive network throughout
a polymer matrix and thus to transform an insulating
polymer into conductive. This critical concentration
primarily depends on the shape of the filler particles
and on their distribution within a given polymer
matrix. The dispersion state of CB particles in a
single polymer depends on factors such as surface
free energy, melt viscosity and size and structure of
the CB particles [1,2].
Although percolative systems can easily become
highly conductive, it is rather difficult to reliably and
reproducibly attain desired intermediate conductivity
levels necessary for ESD applications (106 - 109
ohms/sq). This is due to the steepness of the
resistivity vs. carbon black concentration curve. Static
controlled products usually contain 15 to 20 wt%
carbon black. This relatively high carbon black
concentration has a negative effect on both the
processability of a given compound and on its
mechanical properties.
Immiscible polymer blends have been recently
reported to conduct electricity at a much lower
concentration of a conducting filler than the
theoretical predictions [3]. Such conductive blends
have higher conductivity values than those of their
individual components at the same CB content, and
may achieve also high conductivity levels at lower CB
loadings [4-14]. The electrical conductivity of
conductive CB filled binary polymer blends is
determined by the CB concentration and on its
distribution details, either in one of the phases, at the
interface boundaries, or combination thereof [8,12,
14].
static dissipative injection moldable thermoplastic
composite materials have been recently developed.
These innovative materials are based on combining a
number of polymeric materials with glass fibers and
CB to produce a uniform, multi-component
thermoplastic composite with consistent electrical and
mechanical properties [15,16,17]. The present report
describes property - structure relationships of
polypropylene/nylon/glass
fiber
(PP/PA/GF)
composites with consistent resistivity levels within
the 106 - 109 ohms/sq range achieved at very low
carbon black loadings. These four-component systems
structure
spontaneously
during
the
hot
compounding/processing steps to form an electrically
conductive network of the nylon component
encapsulating the glass fibers, with carbon black
particles located mostly at the nylon/polypropylene
interface and some within the encapsulating nylon
shells. These unique systems, spontaneously
structured in the melt, can be actually described by a
triple-percolation morphology of a continuous glass
fiber network, continuous nylon phase and continuous
carbon black pathways. Such triple-percolation
structures have not been previously reported.
The mechanism of
formation of a segregated
distribution of fillers is explained by the difference in
their affinity to either component constituting the
polymer blend [7]. The localization of CB in an
immiscible polymer blend is basically controlled by
the mutual polymer-polymer and polymer-filler
interactions, although the melt viscosity of the
components can have a kinetic effect on the
thermodynamic process [7, 9]. If the melt viscosities
of two polymers are comparable and within normal
processing levels, the uneven distribution of fillers in
a polymer blend matrix is mainly due to the difference
in affinity of CB particles to each component
constituting the polymer blend [5]. Carbon black
incorporated into polypropylene/nylon immiscible
blends forms segregated structures of CB particles
located at either or both the polypropylene/nylon
(PP/PA) interface and within the nylon phase [21]. In
CB
filled
high
density
polyethylene/poly(methylmethacrylate)
(HDPE/
PMMA) blends, most of the CB particles were found
in the HDPE phase and, especially, at the interface,
whereas in PMMA/PP blends, most of the CB
particles locate in the PMMA phase and also at the
interface of the two polymers [8]. Addition of CB to
high impact polystyrene/linear low density
polyethylene (HIPS/LLDPE) systems is accompanied
by its preferential location in the LLDPE [4].
It should be emphasized that all the reported literature
results are for compression molded samples. Injection
molding severely diminishes the conductivity of
blends that are conductive as compression molded
samples [4]. This is attributed to the high shear levels,
characteristic to injection molding, which deteriorate
the existing conductive networks and cause enhanced
deagglomeration of the CB agglomerates.
To overcome the difficulties associated with injection
molding of CB containing conductive plastics, new
Low carbon black loadings have led to the creation of
a new family of injection moldable ESD composites
with an easy control of resistivity level and also with
the ability to produce materials for clean room
applications.
Experimental
The multi-component thermoplastic composites were
compounded in a co-rotating twin screw extruder
(Berstorff, 25 mm, L:D = 28:1) and subsequently
injection molded using a Battenfeld (80 ton) injection
molding machine equipped with a three cavity ASTM
mold (tensile bar, flexural bar and falling dart impact
disc).
Commercial grades of polypropylene (PP), (Carmel
Olefins, Israel), polyamide-6,6 (PA), (Polyram Israel), and short glass fibers (GF) (Owen Corning,
Vetrotex) were used in this study. A highly structured,
electrically conductive carbon black (CB),
Ketjenblack EC 600 JD (Akzo, The Netherlands), was
used as the conductive filler.
The surface and volume resistivity of injection
molded samples was tested at 10V or 100 V according
to EOS/ESD S11.11 and EIA 541, using a Keithley
6517 instrument connected to a concentric fixture
(guarded ring) (Keithley, Model
8009). Each
reported value is an average of six test specimens.
The blends phase morphology of freeze fracture
surfaces was studied using a Phillips XL20 scanning
electron microscope (SEM).
Dynamic mechanical properties were measured by
dynamic mechanical thermal analysis system
(Rheometrix), in the torsion mode. The frequency
used was 1 Hz and heating was carried out under an
inert nitrogen atmosphere at a rate of 2 oC/min.
The material’s contamination level was evaluated by
extraction in deionized water at 80 oC. The water was
then analyzed for leachable anions and cations by an
inductively coupled mass spectroscopy (Dionex
4500I).
Results and Discussion
Resistivity - Morphology Relationship
The effect of CB content on the volume resistivity
of the individual polymers and of their blend, all
injection molded, is depicted in Figure 1. A
characteristic drop in resistivity, i.e. percolation,
is observed for all the studied systems. PP
percolates at about 4 wt% CB while PA has a
higher percolation threshold and it is less
conductive than PP. The PP/PA (70/30)
compound has a lower CB percolation threshold
than the individual polymers. Noteworthy is the
threshold percolation concentration of the
PP/PA/GF/CB composites which is the lowest, at
about 1 wt% CB, as a result of the unique
morphology spontaneously developed during the
compounding/injection molding processes. Figure
1 shows that the resistivity values of the new
PP/PA/GF/CB material are well within the ESD
range (106 - 109 ohm-cm) at a CB concentration
of 1-1.5 wt%. The resistivity of all the other
polymer systems, at 1-1.5 wt% CB, exceeds 1012
ohm-cm. It is important to note that the CB used
in this study is EC 600 - a high structure CB type.
To achieve similar resistivity levels, higher
concentrations of low structure CB types are
required compared to EC 600 [17].
Log Resistivity, ohm.cm
The ASTM test methods D638, D790 and D256 were
used to determine mechanical properties, ASTM
D648 for thermal properties and ASTM D792 and
D570 for density measurements. Each reported value
is an average of five test specimens.
15
10
5
0
0
2
4
6
Carbon Black, wt%
8
PP/CB
PA66/CB
PP/PA/CB
PP/PA/GF/CB
Figure 1. Volume resistivity of PP, PA 6,6, PP/PA (70/30) and
PP/PA/GF (15% GF) injection molded blends as a function of CB
content.
The CB location within PP, PA, PP/PA blend and the
PP/PA/GF composites is observed in the SEM
micrographs in Figures 2 and 3. There is a significant
difference in CB distribution in PP and PA, where in
PP a chain-like structure of CB agglomerates is
observed, whereas in PA, the CB, at the same content,
is distributed more uniformly. Addition of CB to the
PP/PA blend is accompanied by its preferential
location in the PA, as depicted in Figure 2. This
clearly demonstrates the difference in the interaction
of CB particles with various polymer matrices.
According to Miyasaka et al. [1], the higher the
polarity and the surface tension of a given polymer,
the larger the critical CB content is. Indeed, PA has a
significantly higher surface tension and polarity than
PP. Thus, CB, having a relatively high surface
tension, clusters and forms conducting networks at
low CB contents in non-polar polymers having low
surface tension, such as PP. In a previous paper [5],
PP/PA blends with CB having a co-continuous phase
morphology exhibited appreciable conductivity levels
at low CB contents for PA content of about 30 %, i.e.
double percolation structures were found.
continuity. Addition of GF to the PP/PA/CB blends
transforms an insulating
PP/PA/CB compound
containing less than 2 wt% CB to a conductive
system upon the addition of about 15 wt% GF. The
added GF undergo coating with PA and the CB
locates at the PP/PA interfaces and also within the
encapsulating PA (Figure 3). Thus, the addition of
insulating GF to the PP/PA/CB compound actually
leads to spontaneous in-situ formation of conducting
fibers. The minor polymer affinity to the glass fibers
is a controlling factor in determining the blend’s
morphology. The possible morphologies of ternary
polymer/polymer/filler immiscible polymer blends
range from a separate dispersion of the minor
polymeric component within the continuous matrix, to
encapsulation of the filler particles by the minor
polymeric component. The actual morphology is
developed during the hot blending process and is
influenced by thermodynamic and kinetic effects.
Examples of immiscible blend systems exhibiting
filler encapsulation are PP/PA-6 blends filled with
glass [18], and PP/elastomer blends filled with
calcium carbonate [19] or talc [20]. The location of
the filler particles is governed by the mutual
wettability of the polymers, by the blend preparation
conditions and the relative viscosity of the
components [19]. The interfacial tension between
the components in a polymer/polymer/filler ternary
blend can be varied by changing the filler surface
treatment, or by addition of a compatibilizer.
Thus, the PP/PA/GF/CB four-component systems,
described in the present article, structure
spontaneously
during
the
hot
compounding/processing steps to form an electrically
conductive network of PA coated GF, with CB
particles located mostly at the PP/PA interface. These
unique systems are characterized by a triplepercolation morphology of continuous GF network,
continuous encapsulating PA phase and continuous
CB pathways. Such triple-percolation structures have
not been previously reported.
Figure 2. SEM micrographs of freeze fracture surfaces of PP,
PA and PP/PA with 6 wt% CB.
The conductivity of the PP/PA/GF/CB composites is
determined by the specific CB morphology forming a
Figure 3. SEM micrographs of freeze fracture surfaces of PP/PA/GF and PP/PA/GF/CB at two different magnifications.
The Effect of GF Content
Resistivity curves of PP/PA/GF/CB composites are
depicted in Figure 4 as a function of GF content. The
morphology of freeze fracture surfaces of samples
with various GF contents is depicted in Figure 5 at
two magnifications. Within the studied range of GF
content the resistivity decreases with GF
concentration. The addition of 10 wt% GF appears to
be sufficient to generate continuity of the PA/CB
coated GF within the continuous PP matrix. However,
composites with higher GF contents exhibit more
uniform conductive networks, resulting in lower and
consistent surface resistivity. It should be noted that
the consistency of surface resistivity in the static
dissipative range occurs over a wide range of GF
concentration, thus assuring control of the other
material properties. The tensile strength and modulus,
and the flexural modulus were found to increase with
fiber content, while the notched Izod impact is
practically independent of fiber content [16,17]. In
addition, the heat distortion temperature (HDT)
increases with fiber content, as seen in Figure 4.
The dynamic mechanical properties of PP/PA/GF/CB
composites and of conventional PP/CB compounds
have been studied. Figure 6 depicts the storage
modulus as a function of temperature. The modulus
of the PP/PA/GF/CB composites is higher in
comparison to a typical PP/CB compound and it
increases with increasing GF content. Moreover,
Figure 6 clearly shows that PP/PA/GF/CB composites
have an enhanced heat stability, namely, their
modulus is better preserved at the higher
temperatures.
170
165
7
160
6
HDT, C
Log Resistivity, ohms/sq
8
155
5
15
25
35
GF Content, %
Figure 4. Effect of GF content on resistivity and heat distortion
temperature (HDT) of various PP/PA/GF/CB composites.
Figure 6. Loss modulus of multi-component composites with
different GF loadings.
Figure 5. SEM micrographs of freeze fracture surfaces of PP/PA/GF/CB samples containing 10 wt% GF and 30 wt% GF at two
different magnifications.
Mechanical Properties
Table 1 presents a summary of basic properties of the
new quaternary thermoplastic composites compared
with the conventional PP/CB materials (PP
homopolymer and impact-modified PP). The
electrical
properties indicate that the new
PP/PA/GF/CB materials with 1.2-1.4 wt% CB exhibit
a consistent surface resistivity within the desired
static dissipative range (106 - 109 ohms/sq), while the
conventional PP/CB materials (containing one order
of magnitude higher CB content)
are in the
conductive range (103 - 104 ohms/sq).
The mechanical properties data shown indicate that
the new materials are significantly stiffer and stronger
than PP/CB and with better dimensional stability.
Composites with improved impact resistance can be
obtained by using a modified PP matrix [17]. It is
important to point out that utilization of the
conventional approach for impact modification to the
quaternary systems has failed since the triplepercolation structure was destroyed. Thus, a new
innovative approach has been sought and successfully
implemented.
In addition, the heat distortion
temperature (HDT) of the new multi-component
composite materials is significantly higher than the
HDT values of the corresponding PP/CB compounds.
It is well known that CB as a particulate filler usually
reduces the mechanical strength of thermoplastic
resins and thus lower CB concentrations are an
advantage. Moreover, at high CB concentrations,
present in the conventional compounds, the release of
CB particles against a counter-face, commonly called
“sloughing”, may make such compounds unsuitable
for some applications. It is important to note that the
new ESD composites, with very low CB loadings, are
significantly cleaner and significantly less
“sloughing” than conventional PP/CB compounds
and, therefore, these materials are suitable for
cleanroom applications.
Contamination
Materials utilized in storage boxes and wafer carriers,
in the MR heads or in the disk drive industries should
have the lowest potential of ion contamination,
outgassing and particulate contamination. Each user
company, at least the larger organizations, generate
their own specifications. While there are often
differences between company requirements, some
specific area and test methods are consistent between
similar operations [21]. Many companies appear to
be interested in ions contamination and may do
extraction tests from materials and items to determine
leachable amounts of flourine, chlorine and bromine,
plus nitrates, phosphates and sulfates. Levels are set
for items and materials depending on their application
and proximity to sensitive products.
Table 1. Properties of PP/PA/GF/CB composites compared with conventional PP/CB compounds.
Carbon Black , %
Tensile Strength, MPa
Flexural Modulus, MPa
Izod Impact, notched, J/m
Izod Impact, unnotched, J/m
Gardner Impact, J
HDT, OC
NEW
PP/PA/GF/C m-PP/PA/GF/CB
B
1.2
1.4
45
22
3100
1900
50
100
360
230
4
11
160
150
>10
32
2740
21
>10
24
1290
480
115
72
Surface Resistivity, ohms/sq
106 - 109
103 - 104
103 - 104
106 - 109
CONVENTIONAL
PP/CB
m-PP/CB
Table 2. Contamination levels (leachable anions and cations) of PP/PA/GF/CB composites
Parameter
PP/PAGF/CB
Regular
PP/PA/GF/CB
Clean
A
PP/PA/GF/CB
Clean
B
PP/PA/GF/CB
Clean
C
PP Base
Resin
Typical Ionic
Contamination
Levels
Leachable
Anion
Cl, ppb
SO4, ppb
NO3, ppb
Leachable
Cations
K, ppb
Na, ppb
NH3, ppb
Ca, ppb
Mg, ppb
4881
508
847
962
101
<750
3854
937
90
9920
141
261
106
2588
456
333
58
5896
120
842
3092
45
56
581
< 200
<500
<3000
491
4853
583
3392
601
167
205
642
1527
47
441
1499
948
3008
66
8
937
939
1164
44
90
210
281
-
<180
<2700
<400
-
The concentrations of leachable anions and cations
were investigated for the new PP/PA/GF/CB
composites. Table 2 summarizes the results of the ion
contamination tests for a regular reference material
(not for the cleanroom application) and for clean
room materials; several GF sources (designated in
Table 2 as A, B and C) were evaluated. The PP based
resin is included for reference. Table 2 provides also
some typical levels of ionic contamination. The
results indicate that anion and cation levels in
PP/PA/GF/CB clean (A) material are well within the
specified typical ionic contamination range.
Moreover, it is evident that the “clean materials” have
significantly lower contamination levels than the
regular one and that different ionic contamination
levels were found for GF from different sources.
Thus, it is possible to control the material’s ion
contamination potential by selecting proper materials
and processing conditions.
Conclusions
New multi-component injection moldable conductive
composites have been developed, complying with the
desired ESD range, 106 - 109 ohms/sq. These
composites represent quaternary systems comprising
PP/PA/GF/CB systems. Some impact modified
conductive composites comprise five or even more
components. All these systems undergo spontaneous
well-defined structuring and an in-situ formation of
conductive fibers during the compounding/processing
steps. The morphologies obtained, described as triplepercolation systems, are characterized by a
continuum of insulating GF becoming conducting as a
result of being encapsulated with the PA minor
component and localization of the CB particles mostly
at the PP/PA interfaces. These unique morphologies
require low CB contents characteristic to the
structured conductive composites. Thus, less than 2
wt% CB is required to produce electrically and
mechanically
consistent
conductive
injection
moldable composites with balanced physical
properties for a variety of applications, where ESD
protection and reduced contamination are required.
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