Thermal, mechanical and tribological properties of polycarbonate ...

Page 1 of 9 DOI 10.1515/polyeng-2013-0039 J Polym Eng 2013; x(x): xxx–xxx

Q1: Please supply full name for “N. Win Khun” unless this is the designation by which the author is usually known – journal style is to give the first name in full

N. Win Khun and Erjia Liu*

Thermal, mechanical and tribological properties of polycarbonate/acrylonitrile-butadiene-styrene blends Abstract: Polycarbonate (PC) and acrylonitrile-butadienestyrene (ABS) were blended by varying ABS content. The thermal, mechanical and tribological properties of the PC/ ABS blends were systematically characterized. Increasing ABS content in the PC/ABS blends decreased thermal stability of the blends, as a result of the lower thermal stability of the ABS than that of the PC. Although the tensile strength of the PC/ABS blends apparently decreased with increased ABS content, the PC/ABS blend with 10 wt% of ABS had the highest tensile strength, because of improved processability of the blend. The friction and wear of the ABS measured against a steel ball of 6 mm in diameter were higher than those of the PC. As a result, a higher ABS content in the PC/ABS blends resulted in higher friction and wear of the blends. The scratch results showed that scratching with a 5 mm steel ball generated a scratch with a shorter length and lower depth on the PC than on the ABS, which indicated better scratch resistance of the PC. Therefore, the PC/ABS blend with 50 wt% of PC had better scratch resistance than the ABS, due to the influence of the PC embedded in the blend. Keywords: PC/ABS blend; scratch; tensile strength; wear.

*Corresponding author: Erjia Liu, School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore, e-mail: [email protected] N. Win Khun: School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

1 Introduction Polymer materials are highly valued and used in a wide variety of applications, such as automotive and household appliances, printing machinery, computer peripherals and textile machinery, because they are easy to process and manufacture, lightweight and relatively inexpensive [1]. However, polymer materials have disadvantages, such

as low load-carrying capacity, short running life, and poor heat resistance when they are employed in tribological applications at high speed, heavy load, and high ambient temperature [2]. In addition, wear of polymer materials contributes to a significant financial loss in industry [3]. Polycarbonate (PC) and acrylonitrile-butadiene-styrene (ABS) are well-known engineering thermoplastics. PC is characterized by its excellent dimensional stability, high elastic modulus, high toughness, high impact strength, and difficult processability due to its high melt viscosity. PC is also insensitive to moisture and durable to various weather conditions. ABS is characterized by its notch sensitivity, low cost, poor flame and chemical resistance, and low thermal stability [4]. Blending of PC and ABS can be an efficient way of developing materials with selectively enhanced properties. The advantage of a PC/ABS combination is that ABS can help reduce the cost and improve the processability [5]. In addition, PC/ABS blends can offer the most desirable properties of both materials, such as superior strength and heat resistance of PC and flexibility of ABS. Therefore, PC/ABS blends are one of the most widely used thermoplastics for automotive, electronic and telecommunication applications. Yakut et al. [6] reported that PC/ABS blends could be widely used as gear materials in an open and moisture environment, since the PC/ ABS materials were durable against flame, air, ultraviolet light, and holding low moisture. Although the mechanical properties of PC/ABS blends, such as tensile strength, fracture toughness etc. [7–9] have been widely studied, their tribological characteristics have been insufficiently reported. For example, numerous attempts to connect wear to hardness have been made [10–12]. However, the results are either not meaningful, or even contradictory to other results. Therefore, a fundamental understanding of a correlation between ABS content in PC/ABS blends, and their tribological properties, is essential for successful tribological applications. In this study, PC/ABS blends were prepared with different ABS contents. The thermal, mechanical, and tribological properties of the PC/ABS blends, along with their correlations, were systematically investigated with respect to ABS content in the blends.

Q2: “…in an open and moisture environment…” – might this be “…in an open and high moisture environment…” or “…in an open and low moisture environment…”. Please check and amend as appropriate

Page 2 of 9 2 N.W. Khun and E. Liu: PC/ABS blend

Q4: Please check this addition “... (Tg)”

2 Experimental details The polymers used in this study were commercially available thermoplastic PC and ABS (pellets). All PC/ABS blends were prepared using a Teflon mold by hot pressing (Carver 3856). A thermogravimetric analyzer (TGA TA 2950) was used to investigate the thermal stability of the samples from room temperature (RT, ∼22°C) to 800°C at a heating rate of 10°C/min in a pure nitrogen environment. Differential scanning calorimetric (DSC) measurements were conducted using a thermal analyzer (TA Q200). Two scans from an equilibrium temperature of RT to 225°C at a heating rate of 10°C/min in a pure nitrogen environment, with an interim cooling at a cooling rate of 10°C/min, were performed for each sample in an aluminum pan. The glass transition temperature (Tg) was determined from the inflection point of the heat capacity curve from each second scan. The surface morphology of the samples was studied using surface profilometry (Talyscan 150), with a diamond stylus of 4 μm in diameter and scanning electron microscopy (SEM) (JEOL-JSM-5600LV). The average root-meansquare roughness (Rq) of the samples was determined from three measurements on each sample. The hardness and Young’s modulus of the samples were measured using a micro indenter (CSM MHT) with a spherical diamond tip of 100 μm in diameter at RT. The indentation tests were performed in a load control mode with a total load of 5 N. In each indentation test, the loading and unloading rates and dwelling time at the peak load were 10 N/min, 10 N/min and 10 s, respectively. The hardness and Young’s modulus of the samples were derived using Oliver and Pharr’s method and their average values were taken from 16 indentation measurements carried out at different locations on each sample. The tensile strength of the samples was measured with a universal tensile tester (Instron 5565) at a crosshead speed of 0.01 mm/s. Standard dog-bone shaped samples with a gauge length of 25 mm and a cross-section of 5 mm × 3 mm were used. Five specimens of each sample were tested to get an average tensile strength. The water contact angle of the samples was measured with a sessile liquid drop method (FTA 200). Three measurements on each sample were carried out to get an average contact angle. The wear and friction of the samples were characterized using a ball-on-disc microtribometer (CSM) at RT. A chrome steel (Cr6) ball of 6 mm in diameter was slid on each sample surface in a circular path of 3 mm in diameter, for about 30 m in sliding distance, at a sliding velocity

of 3 cm/s under a normal load of 1 N. The average friction coefficient of each sample was taken from three measurements on the sample. The widths and depths of the three wear tracks on each sample were determined from the cross-sectional profiles of the wear tracks measured using white light confocal imaging profilometry (Nikon L150) and the average values were taken from the three tracks with four measurements per track. The scratch resistance of the samples was diagnosed using a universal mechanical tester (CETR UMT 3) with a chrome steel ball (Cr6) of 5 mm in diameter. The scratch test was performed under progressive loading from 1 to 15 N for about 5 mm in scratch distance at a scratch velocity of 5 mm/min. The scratch tests were repeated three times per sample.

3 Results and discussion Figure 1 shows the TGA results of the PC, ABS and PC/ABS blends with different ABS contents. It is found that the thermal stability of the ABS is much lower than that of the PC, which is indicated by a larger weight loss of the ABS sample than that of the PC sample. In addition, the remaining weight of the PC at a temperature of about 800°C is about 15% larger than that of the ABS, confirming better thermal stability of the PC. As a result, the increased ABS content in the PC/ABS blends leads to decreased thermal stability of the blends. Figure 2 presents the DSC results of the PC, ABS and PC/ABS blends with different ABS contents, from which the Tg (∼145°C) of the PC is significantly higher than that (∼123°C) of the ABS. When the PC is blended with 10 wt%

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Q3: Please supply manufacturers name, town/city, state and country for all reagents, devices and software mentioned throughout

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Page 3 of 9 N.W. Khun and E. Liu: PC/ABS blend 3

Rq values of the PC/ABS blends are mostly lower than that of the PC, which indicates lower surface roughness of the blends, because blending of the PC with ABS improves molding quality of the blends via enhanced processability of the blends and consequently gives rise to better surface finishes of the blends [13]. In Figure 4A–D, ridges are apparently seen on all the sample surfaces, as protruded asperities above the surfaces are obviously found.

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of ABS, the glass transition range of the PC/ABS blend becomes wider, due to the influence of the glass transition of the ABS, although the blend does not significantly show two Tg values. However, a further increased ABS content in the PC/ABS blend to 50 wt% gives rise to a significant appearance of two Tg values in the glass transition region of the blend. Although the two Tg readings are found in the glass transition region of the PC/ABS blend containing 70 wt% of ABS, the lower PC content in the blend significantly depresses the Tg of the blend that is attributed to the PC, as shown in Figure 2. Figure 3 shows the Rq values of the PC, ABS and PC/ ABS blends with different ABS contents measured using surface profilometry. The Rq values of the PC and ABS are about 0.37 and 0.315 μm, respectively. It is found that the

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However, smoother surfaces have a smaller number of protruded asperities. The water contact angles of the PC, ABS and PC/ ABS blends with different ABS contents are presented in Figure 5. The larger water contact angle (∼64.6°) of the PC than that (∼59.6°) of the ABS indicates a lower surface energy of the PC [14, 15]. The water contact angles of the PC/ ABS blends with 10, 30, 50, and 70 wt% of ABS are about 62.3, 57.2, 58.2, and 57.1, respectively, which implies that blending of the PC with ABS promotes the surface energy, probably due to the higher surface energy of the ABS. The influence of surface roughness on the water contact angle of the PC/ABS blends should be taken into account, since a smoother surface gives rise to a smaller contact angle [16, 17]. The similar trends of the water contact angle (Figure 3) and Rq value (Figure 5) of the samples, with respect to ABS content, indicate that the water contact angle of the samples is attributed to their surface roughness. Figure 6 presents the hardnesses and Young’s moduli of the PC, ABS and PC/ABS blends with different ABS contents. The hardness (∼114 MPa) and Young’s modulus (∼1.91 GPa) of the ABS are significantly lower than those of the PC (hardness of ∼206 MPa and Young’s modulus of ∼2.49 GPa). Therefore, the increased ABS content in the PC/ABS blends from 10 to 70 wt% apparently decreases the hardness and Young’s modulus of the blends from about 189 to 138 MPa and from about 2.22 to 1.96 GPa, respectively, as found in Figure 6. In Figure 7, the increased ABS content in the PC/ABS blends from 30 to 70 wt% apparently decreases the tensile strength of the blends from about 58.5 to 46.74 MPa, because the tensile strength (∼45.21 MPa) of the ABS is significantly lower than that (∼58.9 MPa) of the PC. The

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Figure 7 Tensile strengths of polycarbonate (PC)/acrylonitrilebutadiene-styrene (ABS) blends as a function of ABS content.

tensile strength of the PC/ABS blend with 10 wt% of ABS should be lower than that of the PC. However, the higher tensile strength (∼61.03 MPa) of the PC/ABS blend with 10 wt% of ABS implies that blending of the PC with 10 wt% of ABS gives rise to the higher tensile strength via the better miscibility and processability of the blend, since poor interfacial adhesion between the two components in the blend can lower the tensile strength of the blend [15]. Although the increased ABS content in the PC/ABS blends more than 10 wt% can further improve processability of the blends, the subsequently increased influence of the ABS embedded in the PC/ABS blends leads to the decreased tensile strength of the blends. Figure 8 shows the cross-sectional morphologies of the fractured PC, ABS and PC/ABS blends with different ABS contents after the tensile tests, where the direction of the tensile testing is perpendicular to the fracture morphologies of the samples. In Figure 8A, fracture lines are


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Figure 8 Scanning electron microscopy (SEM) micrographs showing cross-sectional views of (A) polycarbonate (PC), PC/acrylonitrile-butadiene-styrene (ABS) blends with (B) 10, (C) 50 and (D) 70 wt% of ABS, and (E) ABS observed after tensile tests. The insets in (A, C, D, and E) show cross-sectional views of the same samples at higher magnifications.

significantly found on the cross-sectional morphology of the PC, resulting from shear fracture of the PC caused by initiation and propagation of cracks through planes, perpendicular to the direction of the tensile testing, weakened by porosities [18–20]. Although blending of the PC with 10 wt% of ABS does not significantly change the fracture morphology of the blend as shown in Figure 8B, the denser fracture lines are observed on the crosssectional morphology of the PC/ABS blend with 10 wt%

of ABS, because the fracture has propagated through ABS domains existing in continuous PC component of the blend [18–20]. Increasing the ABS content in the PC/ ABS blends through 50–70 wt% apparently change the fracture morphology of the blends, as found in Figure 8C and D. In Figure 8E, the craze fracture of the ABS probably results from cavitation (voiding) of rubber particles in the ABS matrix during the tensile test [18–20]. Therefore, the developed continuous ABS component and decreased

Q5: Might this be “…in the continuous PC component…” or “…in continuous PC components…” – please check and amend if appropriate


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PC domains in the PC/ABS blends with increased ABS content apparently change the fracture morphology of the blends through a significant progression in fracture mode from the shear fracture of the PC to the craze fracture of the ABS [18–20]. Figure 9A and B present the friction coefficients of the PC, ABS and PC/ABS blends with different ABS contents, slid against 6 mm steel balls for about 30 m in sliding distance under a normal load of 1 N, as functions of ABS content and sliding distance, respectively. As shown in Figure 9A, blending of the PC with 10 wt% of ABS gives rise to a lower friction coefficient (∼0.08) than that (∼0.31) of the PC. The lower friction coefficient of the PC/ABS blend with 10 wt% of ABS can be correlated to its increased tensile strength (Figure 7), because the increased tensile strength of the PC/ABS blend reduces friction by effectively preventing wear of the blend during the sliding. However, no correlation between the friction coefficient (Figure 9) and hardness (Figure 6) of the PC/ ABS blend with 10 wt% of ABS indicates that the tensile strength of the PC/ABS blend is more important to the tribological performance of the blend. In Figure 9A, the ABS shows the highest friction coefficient, about 0.47, among the samples used in this study, because the lowest mechanical strength of the ABS (Figures 6 and 7) results in the highest wear. Therefore, the reduced mechanical strength of the PC/ABS blends with increased ABS content gives rise to the increased friction coefficient of the blends from about 0.13 to 0.36 with increased ABS content from 30 to 70 wt%, through the promoted wear of the blends. Since a rougher surface can generate a higher friction, the effect of surface roughness on the friction of the samples should be taken into account [21–23]. However, the trends of the Rq value (Figure 3) and friction coefficient (Figure 9A) of the PC/ABS blends with respect to ABS content are not similar, which implies that the influence of the surface roughness on the friction of the PC/ABS blends is not significant in this study, due to the stronger effect of the mechanical strength of the blends. In Figure 9B, the sliding of the steel ball on the PC increases the friction coefficient to about 0.35, by promoting the initial wear of the rubbing surfaces before the significant wear of the PC surface at about 13.5 m, which is indicated by a large fluctuation in the friction coefficient of the PC centered at about 0.45. When the steel ball is slid on the ABS, the friction coefficient of the ABS abruptly increases to about 0.45 at about 4.5 m and fluctuates at about the same value for the rest sliding distance. However, blending of the PC with 10 wt% of ABS does not generate any severe wear throughout the wear test, which is confirmed by the stable frictional behavior of the

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Figure 9 Friction coefficients of polycarbonate (PC)/acrylonitrilebutadiene-styrene (ABS) blends, slid against 6 mm steel balls in a circular path of 3 mm in diameter for about 30 m in sliding distance, at a sliding velocity of 3 cm/s under a normal load of 1 N, as functions of (A) ABS content and (B) sliding distance. (C) Widths and depths of the wear tracks on the same samples tested under the same conditions mentioned above.

PC/ABS blend (Figure 9B). The increased ABS content in the PC/ABS blend to 30 wt% still does not result in any severe wear during the wear test, but gives rise to a linear increase in the friction coefficient with increased sliding distance, due to the promoted wear of the blend. When the ABS content in the PC/ABS blend reaches 50 wt%, the


sliding of the steel ball on the PC/ABS blend leads to a significant wear of the blend at about 19.5 m, indicating that 50 wt% of ABS in the PC/ABS blend degrades the wear resistance of the blend. It is consistently found that further increase in ABS content in the PC/ABS blend to 70 wt%, gives rise to the earlier surface breakdown of the blend at about 11 m. The similar trends between the friction (Figure 9A) and wear (Figure 9C) of the PC/ABS blends versus ABS content, confirm that the friction of the PC/ABS blends is closely related to the wear resistance of the blends. Figure 10 shows the surface morphologies of the worn PC, PC/ABS blends with different ABS contents, and ABS after sliding against 6 mm steel balls for about

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30 m sliding distance, under a normal load of 1 N. In Figure 10A, severe plastic flow is found on the surface of the PC slid against the steel ball. The is due to the sliding of the steel ball on the PC generating high frictional heat, which in turn causes softening of the sample, so the softened surface readily flows under the repeated sliding [24]. In Figure 10B, blending of the PC with 10 wt% of ABS significantly reduces the wear of the blend, because the increased mechanical strength of the blend (Figure 7) effectively prevents the removal of material during the wear test. Abrasive marks on the wear track of the PC/ ABS blend with 10 wt% of ABS show that the wear of the PC/ABS blend is attributed to the abrasive wear caused by the rubbing of the steel ball. However, 70 wt% of ABS

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Figure 10 Surface morphologies of worn (A) polycarbonate (PC), PC/acrylonitrile-butadiene-styrene (ABS) blends with (B) 10 and (C and D) 70 wt% of ABS and (E and F) ABS after sliding against 6 mm steel balls in a circular path of 3 mm in diameter for about 30 m in sliding distance, at a sliding velocity of 3 cm/s under a normal load of 1 N.


in the PC/ABS blend results in severe abrasive wear of the blend, as shown in Figure 10C and D. In addition, the removal of materials from subsurface regions as platelets (Figure 10D) indicates that the repeated sliding of the steel ball on the sample surface under a high normal load generates surface fatigue wear of the sample. Such surface fatigue wear is not found on the wear track of the PC, indicating high resistance of the PC to fatigue wear (Figure 10A). No observation of the fatigue wear on the wear track of the PC/ABS blend with 10 wt% of ABS (Figure 10B) is a result of the stronger influence of the PC embedded in the blend. The ABS slid against the steel ball has the highest wear attributed to its lowest mechanical strength, as shown in Figure 10E and F. Severely ploughed furrows on the surface and the material removal from the subsurface of the ABS clearly show that the ABS used is very susceptible to abrasive and fatigue wear during the sliding contact with the steel ball. Therefore, increasing ABS content in the PC/ABS blends apparently reduces wear resistance of the blends when the influence of the ABS embedded in the blends takes effect. The polymer with poor thermal stability cannot bear high frictional heat generated due to the lack of heat dissipation within the polymer during sliding, so the resulting degradation of the polymer surface leads to an easy removal of the surface material. It is clear that the lowered thermal stability of the PC/ABS blends with increased ABS content (Figure 1) contributes to the increased wear of the blends, by pronouncing the surface degradation of the blends during the sliding. A possible adhesion between two surfaces in contact can contribute to the wear of the polymer [23, 25]. Although a systematic correlation between the surface energy (Figure 4) and tribological performance (Figure 9) of the PC/ABS blends is not found, it is supposed that the higher surface energy of the PC/ABS blends than that of the PC would partially contribute to the higher wear of the blends, by promoting the adhesive wear. The PC, ABS and PC/ABS blend with 50 wt% of PC were further scratched using 5 mm steel balls for about 5 mm in scratch distance, under progressive loading from 1 to 15 N. Normally, a contact between a sharp tip and a sample during scratching induces a cutting state between them, which results in removal of material from the sample surface [26, 27]. It is supposed that a contact between the 5 mm steel ball and sample gives a plowing or wedge formation state instead of giving a cutting state between them. In Figure 11, the surface topographies of the scratched samples clearly show that the scratching of the steel balls on the samples causes plowing or wedge formation on the surfaces. In addition, no apparent damages

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Figure 11 Surface topographies of (A) polycarbonate (PC), (B) PC/ acrylonitrile-butadiene-styrene (ABS) blend with 50% of PC and (C) ABS scratched with 5 mm steel balls for about 5 mm in scratch distance, at a scratch velocity of 5 mm/min under progressive loading from 1 to 15 N. The insets (top left) in (A–C) are optical images showing surface morphologies of the same samples. The inset (bottom right) in (C) shows surface topography of the ABS measured using surface profilometry.

along the scratches of the samples confirm that the scratching of the steel balls on the samples does not generate a cutting state between the steel balls and samples. A comparison between Figure 11A and C clearly shows that the PC has the shorter scratch than the ABS, indicating that the PC has better scratch resistance. Therefore, the PC/ ABS blend with 50 wt% of PC has the shorter scratch than the ABS, due to the reinforcement of the PC component in the blend. The scratch depths of the samples at an applied load close to 15 N (inset in Figure 11C, bottom left) were measured to further evaluate the scratch resistance of the samples. It is consistently found that the scratch depth of the ABS (16 ± 2 μm) is larger than that of the PC (11 ± 2 μm) and the PC/ABS blend with 50 wt% of PC (13 ± 1 μm). It can


be deduced that blending of the ABS with 50 wt% of PC improves the scratch resistance of the PC/ABS blend more than that of the ABS.

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4 Conclusions In this study, the thermal, mechanical and tribological properties of PC/ABS blends were systematically investigated with respect to ABS content in the blends. – The thermal stability of the ABS was significantly lower than that of the PC, so increasing ABS content in the PC/ABS blends apparently reduced the thermal stability of the blends. – The hardness and Young’s modulus of the PC/ABS blends significantly decreased with increased ABS content, since the hardness and Young’ modulus of the ABS were lower than those of the PC. It was consistently found that the tensile strength of the PC/ ABS blends apparently decreased with increased ABS content. However, blending of the PC with 10 wt% of ABS gave rise to a higher tensile strength of the PC/ ABS blend than that of the PC, because the enhanced processability of the PC/ABS blend resulted in better quality of the blend.

Q6: Please confirm full page range for all references

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The frictional behavior of the PC/ABS blends slid against 6 mm steel balls was closely related to their wear behavior, which was confirmed by the similar trends of the friction and wear of the blends with respect to ABS content. The wear resistance of the PC/ ABS blends apparently decreased with increased ABS content, due to the lower wear resistance of the ABS than that of the PC. However, blending of the PC with 10 wt% of ABS resulted in the highest wear resistance among the blends used in this study, even higher wear resistance than that of the PC due to improved mechanical strength of the blend. The PC sample scratched with a 5 mm steel ball showed a scratch with shorter length and smaller depth than the ABS, which implied better scratch resistance of the PC sample. As a result, the PC/ABS blend with 50 wt% of PC had a higher scratch resistance than the ABS sample, due to the reinforcement of the PC component in the blend. The results clearly showed that PC/ABS blend with 10 wt% of ABS had the highest tensile strength and lowest friction and wear.

Received February 24, 2013; accepted June 18, 2013

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