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: aaytac@kocaeli.edu.tr

Phone: +90 262 303 35 32

Current address: Chemical Engineering Department, Kocaeli University, Engineering

Faculty, 41380 Izmit/Kocaeli TURKEY

2

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

3

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

4

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

5

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.

6

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

7

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)

8

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

9

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

11

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

12

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

13

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;

14

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).

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7