Short carbon fiber reinforced polycarbonate composites: Effects of different sizing materials
Transcript of Short carbon fiber reinforced polycarbonate composites: Effects of different sizing materials
Accepted Manuscript
Short Carbon Fiber Reinforced Polycarbonate Composites: “Effects of Different
Sizing Materials”
Cem Ozkan, N. Gamze Karsli, Ayse Aytac, Veli Deniz
PII: S1359-8368(14)00109-7
DOI: http://dx.doi.org/10.1016/j.compositesb.2014.03.002
Reference: JCOMB 2960
To appear in: Composites: Part B
Received Date: 30 July 2013
Revised Date: 27 February 2014
Accepted Date: 5 March 2014
Please cite this article as: Ozkan, C., Gamze Karsli, N., Aytac, A., Deniz, V., Short Carbon Fiber Reinforced
Polycarbonate Composites: “Effects of Different Sizing Materials”, Composites: Part B (2014), doi: http://
dx.doi.org/10.1016/j.compositesb.2014.03.002
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Short Carbon Fiber Reinforced Polycarbonate Composites: “Effects of Different
Sizing Materials”
Cem Ozkan1, N. Gamze Karsli2, Ayse Aytac1,2*, Veli Deniz1,2
1Department of Polymer Science and Technology, Kocaeli University,
2Department of Chemical Engineering, Kocaeli University, Engineering Faculty, 41380,
Kocaeli/Turkey
*Corresponding author: Ayse AYTAC
E-mail: [email protected]
Phone: +90 262 303 35 32
Current address: Chemical Engineering Department, Kocaeli University, Engineering
Faculty, 41380 Izmit/Kocaeli TURKEY
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Abstract
In this paper, effects of the sizing material type and level on the mechanical,
electrical and morphological properties of the short carbon fiber (CF) reinforced
polycarbonate (PC) composites were investigated. Unsized CF and the CFs which were
sized with epoxy/phenoxy(EPO_PHE), polyimide(PI) and phenoxy(PHE) were used as
reinforcement materials. Fiber length distribution analysis indicated that sizing
protected the CFs breakage into the smaller lengths during the processing. Effects of the
sizing material type and level on the mechanical properties of CF reinforced PC
composites were investigated by means of tensile and izod impact strength tests.
Tensile test results revealed that tensile strength and modulus values of sized CF
reinforced PC composites were higher than that of unsized CF reinforced PC
composites. Besides, effect of the sizing material level on the tensile properties of
composites changed with respect to the sizing material type. It was also found that the
measured effects of the sizing agent type and level on the notched izod impact strength
of composites were not so significant. In addition to this, it was found that sized CF
reinforced PC composites had higher electrical conductivity values than unsized CF
reinforced PC composites. Also, PHE sized CF reinforced composites had the highest
electrical conductivity value among the other composites. Better interactions between
EPO_PHE and PHE sized CF and PC matrix were observed from the scanning electron
microscope analysis. As a result of this study, PHE and EPO_PHE sized CFs can be
suggested as proper reinforcements for PC matrix.
Keywords: A. Carbon fibre; A. Polymer-matrix composites (PMCs); B. Mechanical
properties
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1. INTRODUCTION
Carbon fibers (CFs) have been widely used as reinforcement materials in
composite manufacturing due to their exceptional properties such as high specific
modulus, strength, stiffness, electrical properties and low density. While chemical and
thermal properties of composites mainly depend on matrix materials, mechanical
properties of composites such as strength depend on properties of carbon fiber and
fiber/matrix interfacial adhesion strength. If there is good fiber/matrix adhesion
strength, the applied load can be transferred from matrix to fiber more efficiently [1].
However, generally adhesion between carbon fiber and thermoplastic polymer matrix is
poor because of the inert characteristics of fiber surface and matrix material [2,3].
Polycarbonate (PC) is one of these polymers and it is used in the short fiber reinforced
advanced composites [4]. Different modification techniques have been applied to fiber
surface to improve the interfacial adhesion between thermoplastic matrix and carbon
fiber due to the lack of adhesion between them [1,2,4,5]. Plasma oxidation, radiation
and chemical treatments are some of various methods which were applied to carbon
fiber surface [6-13]. Another efficient method for fiber surface modification is sizing or
coating of fiber surface with a thin polymeric layer. Sizing method prevents the fiber
from the breakage during filament winding, prepreg, weaving and other composite
manufacturing processes [7,8]. This method also improves the interfacial adhesion
between fiber and matrix, since sizing material includes functional groups which can
react with constituents of composite [2]. Besides, chemical structures of the sizing
material and matrix should be similar due to the ‘‘similar dissolve mutually theory’ [6].
Consequently, different matrix materials require proper sizing materials and it is
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important to choose the ideal sizing material for polymer matrix material in order to
obtain better composite properties.
To our knowledge, there has been no study in which the effects of the sizing agent
type and/or level on the properties of carbon fiber reinforced polycarbonate composites
were investigated together. But there are a few studies which are investigated the
adhesion between CF and PC matrix. Dányádi et al. [4] studied the effects of four
different coupling agents on the properties of CF reinforced PC composites. These
coupling agents were containing epoxy, anhydride and isocyanate functional groups.
They found that while the coupling agents which contain epoxy and isocyanate reacted
with PC matrix, coupling agents which contain anhydride functional groups did not.
Kushawa et al. [14] studied the properties of polycarbonate composites which were
reinforced with nickel coated carbon fiber. Their results showed that tensile, flexural
properties and abrasion resistance of composites improved with the surface coating of
fibers. Kim et al. [15] analyzed the effects of fiber length, fiber content, screw speed
and fiber sizing on rheological and mechanical properties of polycarbonate/carbon fiber
composites. They showed that the final fiber length of sized carbon fibers was greater
than that of unsized carbon fibers after the extrusion process and they concluded that the
final fiber length has a strong effect on the rheological and mechanical properties of
composites. Raghavendran and Drzal [3] investigated the adhesion between PC matrix
and CF on which two types of polymer was grafted to create covalent linkages. These
polymers were low molecular weight PC and PMMA. They performed interfacial shear
strength adhesion measurements and observed that the level of interfacial adhesion was
improved by using polymer grafted carbon fiber in the composites. Their results showed
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that the improvement in interfacial adhesion was ranged from %20 to %80, when
polymer grafted carbon fibers were used in composites.
In addition to these studies, there are some studies in the literature which
investigated the effects of sizing agent properties such as molecular weight and
concentration. Zhang et al [2,16] studied the influence of different molecular weight
sizing agents on the properties of carbon fibers and their composites. Their study
revealed that interfacial shear strength and hydrothermal ageing decreased in the case
where high and low molecular weight sizing agent was used. On the other hand,
interfacial shear strength and hydrothermal ageing improved when moderate molecular
weight sizing agent was used. In another study [17], the effect of sizing agent
concentration on the performance of CF reinforced epoxy based composites was
investigated. In this study three levels of sizing agent concentration were studied and it
had been found that the optimum level of sizing agent was 1.5 wt %.
In this study, it was aimed to investigate the effects of sizing material type and
level on the properties of carbon fiber reinforced polycarbonate composites. For this
purpose, unsized and CFs which were sized with three different types of sizing agent
were used as the reinforcement material. Tensile test and izod impact test were carried
out to investigate the effect of sizing material type and level on mechanical properties of
carbon fiber reinforced PC composites. In addition to this, thermal stability of sizing
materials was investigated by thermogravimetry analysis (TGA). Optical microscope
analysis was performed to determine the fiber length distrubition. Scanning electron
microscope (SEM) was also used to analyze the fracture surfaces of composites.
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2. MATERIALS AND METHODS
Polycarbonate (Wonderlite PC 110) was used as matrix material and supplied
from Kempro (Istanbul). PAN-based carbon fibers (Aksaca), which were unsized and
sized with three different materials, were supplied by Akkök Group (Turkey) and used
as the reinforcement materials. Sizing materials were epoxy/phenoxy (EPO_PHE),
phenoxy (PHE) and polyimide (PI). Level of sizing materials on CF surface were 1%,
2% and 3% by wt. Carbon fiber content in composites was kept constant at 30% by wt
and the initial fiber length was 6.0 mm. Composites were prepared by using a laboratory
scale DSM Xplore micro-compounder at 295 °C processing temperature, 100 rpm screw
speed and 3 min mixing time. After the extrusion process, all compounds were molded
by using a laboratory scale injection molding machine (DSM Xplore 12 ml Micro-
injection Molder) with 295 °C barrel temperature, 100 °C mold temperature; and 10
bars injection pressure.
Samples were burned in an ash oven for 30 min at 600 °C to investigate the fiber
length distribution of composites. The residual ash was dispersed in water and then CFs
were isolated from the composite. After that CFs were transferred to glass slides and
images of fibers were obtained from optical microscope. These images were analyzed
by using Image J® in order to determine the fiber length distribution.
Tensile properties were investigated according to ISO-527 by using Shimadzu 100
kN model universal testing machine. Dimension of the test samples were 10 mm width,
4 mm thickness and 106 mm length. Tensile strength at yield and strain at break values
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of composites were determined by using 5 mm/min crosshead speed. Notched izod
impact strength test was performed according to ISO 180/1A by using a Ceast machine
with a 5.5 J hammer and 3.46 m/s impact velocity. After these tests, average of five
measurements was reported with standard deviations.
Thermogravimetric analysis (TGA) was conducted to determine the thermal
stability of CFs which were unsized and 3 wt % sized with different sizing materials at
the processing temperature. Thermal analysis was performed by using a Perkin Elmer
TGA Instrument. For isothermal TGA study, temperature was increased from 30 °C to
295 °C as quickly as possible and held at this temperature for 3 min under atmospheric
conditions similar to conditions during composite preparation. Weight loss data at
constant temperature were collected as a function of time by using special software.
Electrical resistivity values of composites were measured with 2-point-probe
technique by using Haoyue M890G Digital Multi Meter. For a good electrical contact in
two point probe technique, copper wires were attached to both ends of the test specimen
with silver paste. Resistivity measurements were performed by contacting probes with
these copper wires, after the hardening of silver paste. After obtaining the resistance
values of each sample, electrical conductivity values of composites were calculated
[18,19] and average results of five measurements were reported for each prepared
composite:
)( Resistance)(cm Area Electrode
(cm) Thickness Sampleσ(S/cm)
2 Ω×= (1)
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Tensile fractured surface morphology of composites was observed by using
scanning electron microscope (SEM) (JEOL JSM-6510). All the sample surfaces were
sputter coated with gold and palladium before the observation.
3. RESULTS AND DISCUSSION
3.1 Fiber Length Distribution
It is well known that, in the extrusion process, an excessive amount of shear
stress is applied to composite during composite preparation and then composite is
transferred to injection machine for molding. Meanwhile, fibers in the composite are
broken and the fiber length distribution in the composite changes [20,21]. In this study,
the effect of sizing material on the ultimate fiber length distribution in PC composites
was investigated by using Image J® analyzing program. Ultimate fiber length
distribution curves on number average basis for unsized and sized CF reinforced PC
composites were given in Figure 1. It can be seen from Figure 1 that the measured
ultimate fiber lengths are in the range from 25 to 500 μm. Results of the fiber length
distribution analysis shows that while number average fiber length is found to be 91 μm
for unsized and PI sized CFs reinforced composites, it is 100 μm for EPO-PHE sized and
PHE sized CFs reinforced composites. Moreover, after 50 μm fiber length, the highest
number of fibers is observed for PHE sized CF reinforced composite among the other
composites. In addition, in the range of 0-25 μm fiber length, while the maximum
number of fiber is observed for unsized CF reinforced composites, PHE sized fibers
give the lowest number of fiber. This means that unsized carbon fibers can be broken
into smaller pieces than PHE sized carbon fibers; because, the bonding force between
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PHE sized carbon fiber and PC matrix may be higher than that of unsized carbon fibers
and PC matrix. This might also be due to the protection effect of the sizing materials on
fiber breakage. Similar results have been reported by Kim and et.al. [15].
3.2 Tensile Test of Composites
Effect of the sizing material type and level on tensile strength of CF reinforced PC
composites were given in Figure 2. It can be seen from Figure 2 that addition of CF to
the neat PC matrix (58.7 MPa) increases the tensile strength as expected. Besides,
tensile strength values of sized carbon fiber reinforced PC composites are always higher
than that of unsized CF reinforced composite. This figure also shows that PHE and
EPO_PHE sized carbon fiber reinforced composites have higher tensile strength values
than PI sized carbon fiber reinforced composites. Tensile strength values of the
composites are affected by the fiber-matrix interaction at the interface. If interaction
between fiber and matrix is poor, fibers easily separate and pull out from the matrix. If
fibers are sized with proper sizing material which can interact with the matrix, this
interaction hinders the separation and pull out of fibers from the matrix. In our study,
higher tensile strength values and number average fiber lengths with PHE and
EPO_PHE sized CF reinforced composites, can be attributed to the better interaction
between fiber and matrix. This better interaction may result from the transesterification
reactions between PC and phenoxy which takes place at the temperatures higher than
230 °C. As a result of these reactions, graft or cross linked copolymers occur and these
copolymers act as a bridge between fiber surface and polymer, thus provide better stress
transfer and result in better tensile properties [22,23].
10
Tensile modulus of composites with respect to the sizing agent type and level
were given in Figure 3. It can be seen in this figure that, tensile modulus of neat PC
matrix (3.2 GPa) increases with the addition of carbon fibers as expected. The amount
of increment can be estimated by the rule of mixture theory. According to this theory;
modulus of a composite is given in the equation below [24]:
mmff01c EVEVE +ηη= (2)
In this equation, η1 is the correction factor and it is used for lengths of fiber
which are not fully contributing to the stiffness of the composite, η0 is the fiber
orientation factor, E is the tensile modulus, V is the volume fraction and subscripts c, f
and m represent composite, fiber and matrix, respectively [20,25]. In our study, fiber
orientation factor η0 was taken as 1 because fibers are aligned along the flow direction
during the injection molding process. Since Em, Vm, Ef and Vf values were constant and
the same for all composites, tensile modulus was considered to be dependent on the
change in η1 values of composites. In this study, tensile modulus values of sized carbon
fiber reinforced PC composites are higher than that of unsized carbon fiber reinforced
composites. This difference may be due to the lower average fiber length or η1 value of
unsized CF reinforced composites. In addition, PHE and EPO_PHE sized CF reinforced
PC composites have the highest tensile modulus (see Fig 3), resulting from the highest
number average fiber length or η1 value.
Effects of the selected sizing material level on the properties of carbon fiber
reinforced composites are different. While low level of sizing material on CFs may lead
to weak coating performance, high level of sizing material may enhance the moisture
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adsorbing capacity of the sizing material and this makes the process ability of CFs
worse. Therefore, sizing material type and level plays a key role on interfacial adhesion
[17]. Results of the sizing agent level on tensile strength and modulus values of PC
matrix composites were given in Fig 2-3. It can be seen from these figures that, tensile
strength value of EPO_PHE sized CF reinforced composites reached to the highest
value at 1 wt% sizing level, on the other hand, PHE and PI sized CF reinforced PC
composites reached to the highest values at 2 wt% sizing level. The highest modulus
values were observed at 3 wt % sizing level for all sizing material type. As seen from
figures, effect of sizing material level on tensile properties of composites changes
depending on the sizing material type.
3.3 Notched Izod Impact Test of Composites
High impact strength properties of PC are based on the carbonate groups which
exist in its structure and these groups provide high flexibility to PC. Notched izod
impact strength of neat PC (11.81 kj/m2) is higher than that of its composites [26].
Generally it can be said that the addition of fiber into the ductile polymer matrix makes
it brittle and this leads to decreasing the impact strength of fiber-reinforced composites
[27-30]. Notched izod impact strength of sized and unsized carbon fiber reinforced
polycarbonate composites was given in Fig. 4. As it can be seen from Fig.4, notched
izod impact strength value of neat PC decreases with addition of CF. Besides, while the
highest notched izod impact strength value for EPO_PHE carbon fiber reinforced
composites is observed at 2 wt % sizing level, the highest notched izod impact strength
value for PHE sized carbon fiber reinforced composites is observed at 1 wt % sizing
level respectively. In addition to this, notched izod impact strength values of EPO_PHE
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and PHE sized CF reinforced composites increased as 15% and 6 %, respectively, when
compared with unsized CF reinforced composite. It can be inferred from these results
that the effect of sizing agent type on the notched izod impact strength of composites, is
not so significant.
3.4 Thermogravimetric Analysis (TGA) of Carbon Fibers
Isothermal TGA was conducted to determine the weight loss of sizing material on
the carbon fiber surface during processing. TGA thermograms of unsized and sized
carbon fibers under isothermal conditions are given in Figure 5. It can be seen from
Figure 5 that while the weight loss percentage of PI sizing material is measured as 0.1
wt%, this value changed for EPO/PHE and PHE sizing materials as 0.8 and 0.7 wt%
respectively. In our previous study we investigated the thermal stability of these sizing
materials on carbon fiber surface by using non isothermal TGA [27]. We have found
that while weight loss percentage of PI sizing was 2 wt%, weight loss percentage of
EPO/PHE and PHE sizing materials were 7 and 10 wt% respectively until 450 0C.
According to these results, it can be concluded that the weight loss during this
processing, in other words, at processing temperature and for this cycle time, is
negligible. It means that our processing condition has no effect on the interfacial
adhesion between fiber and matrix. This result is also confirmed by mechanical test
results of PC composites. Besides, it can be seen from Figure 5 that a weight increase
for unsized CF is seen at processing temperature and cycle time. This weight increase is
explained in the literature as a result of the chemical reaction tendency of oxygen with
aliphatic side groups on carbon fiber surface under atmospheric conditions, during
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processing [31]. It can be inferred from this result that the sizing process is also an
efficient method for protection of carbon fiber surface from oxidation.
3.5 Electrical Resistivity Measurement
Polycarbonate exhibits outstanding electrical insulation property and this property
makes polycarbonate and its composites a prime material for electrical and electronic
components. For this reason, effects of sizing agent type on the electrical properties of
carbon fiber reinforced PC matrix composites were evaluated in this study. As it is
known, electrical conductivity of CF reinforced composites increase with increasing
amount of CF in those particular composites. Electrical behavior of polymer
composites alter from insulator to semi-conductor at critical CF loading level and this
point is known as ‘percolation threshold’. After this point, a continuous network of CF
forms along the polymer matrix and allows the transition of the electrons from one CF
to other by over-crossing the gap between fibers [26]. Electrical conductivity values of
composites are given in Figure 6. It can be seen from Figure 6 that electrical
conductivity values of composites are about 10-3 S/cm. This value is surprisingly higher
than expected. This case may be explained as a result of the conduction mechanism of
fibers which are comes into the contact with each other in the PC matrix. In other
words, it can be concluded that our CF loading level (30 % wt) is higher than
percolation threshold point of PC composites [19].
We have found that sized CF reinforced PC composites have higher electrical
conductivity than unsized CF reinforced PC composites. Moreover, in the case PHE
sized carbon fiber, electrical conductivity values of composite reaches to the maximum;
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Electrical conductivity values of EPO_PHE sized carbon fiber reinforced PC
composites are slightly lower than that of PHE sized carbon fiber filled composites. On
the other hand, PI sized carbon fiber has lower effect on the electrical conductivity
when compared with EPO_PHE and PHE sized carbon fibers. Choi et al. found that
while electrical conductivity of CF reinforced phenolic resin composites did not depend
on the surface treatment methods, it depends upon the dispersion of carbon fibers in the
matrix [19]. The bonding force between unsized CF and PC matrix was lower than that
of sized CF and PC matrix. So in the case of unsized CF, phase separation and
agglomeration occurs in matrix [15]. Therefore the ultimate fiber length is smaller for
unsized CF reinforced composites. In our study, since EPO_PHE and PHE sized carbon
fiber reinforced composites exhibit the longer ultimate fiber length distribution than
other composites; electrical conductivity values of these composites were higher than
that of other composites.
3.6 Scanning Electron Microscopy
Interface studies were carried out to investigate CF surfaces, CFs pull out and
CFs–PC interface by using SEM analyzes. SEM micrographs of the tensile fracture
surfaces of unsized and sized carbon fiber reinforced composites were given in Figure
7(a-d). It can be seen from figure that in case of unsized and PI sized carbon fiber, fiber
surfaces were clean and smooth. Besides, pullout of fibers from the matrix can be seen
from these figures for unsized and PI sized CF reinforced composites. This can be
interpreted as an evidence for poor adhesion between unsized or PI sized carbon fiber
and PC matrix. On the other hand, it can be seen from Figure 7(b,c) that fibers are
covered with a polymeric layer. This case can be attributed to the better interaction
15
between EPO_PHE and PHE sized carbon fiber and PC matrix. This better interfacial
interaction increases mechanical and electrical properties of composite which was
confirmed by the results of performed tests.
4. CONCLUSION
In this study, effect of different sizing material on the properties of CF
reinforced PC composites was studied. Mechanical, electrical and morphological
properties of prepared composites were investigated. Besides, thermal stability of sizing
materials was determined at processing conditions by using isothermal
thermogravimetric analysis (TGA). As a result of the fiber length distribution analysis,
we have found that carbon fibers were protected by sizing materials during processing.
Also, EPO-PHE and PHE sized CFs remained longer than unsized carbon fibers in the
polymer matrix after processing. It was observed that tensile strength and modulus
values of sized carbon fiber reinforced PC composites were higher than tensile strength
and modulus values of unsized carbon fiber reinforced PC composites. Also, effect of
sizing material level on the tensile properties of composites changed with respect to
used sizing material type. Notched izod impact strength results showed that the effect of
sizing agent type and level on the notched izod impact strength of composites was not
so significant. Isothermal TGA analysis results showed that the weight loss amount of
sizing materials was negligible and this means that sizing materials are thermally stable
at the processing conditions. According to electrical resistivity test results, sized CF
reinforced PC composites have higher electrical conductivity values than unsized CF
reinforced PC composites. In addition, PHE electrical conductivity value of sized CF
reinforced composites was the highest among the other composites. It has been seen
16
from SEM micrographs that, while unsized and PI sized carbon fibers pulled out from
the matrix, there was a better interaction between EPO_PHE and PHE sized carbon
fibers and PC matrix. Thus, it can be inferred from these results that PHE and
EPO_PHE sized CFs are more proper reinforcing materials for PC matrix.
Acknowledgement
This study supported by Ministry of Science Industry and Technology of Turkey under
the project number 01020.STZ.21011-2.
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Figure Captions
Fig 1. Fiber length distribution of PC/CF composites
Fig 2. Effect of sizing agent type and level on the tensile strength values of PC/CF
composites
Fig 3. Effect of sizing agent type and level on the modulus values of PC/CF composites
Fig 4. Effect of sizing agent type and level on the notched impact strength values of
PC/CF composites
Fig 5. TGA curves of carbon fibers coated with different sizing agents
Fig 6. Electrical conductivity of the composites
Fig7. SEM micrographs of tensile fracture surfaces of PC/CF composites, (a) Unsized
(x1000), (b) EPO_PHE (x1000), (c) PHE (x1000), (d) PI (x1000).