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polypropylene composites
Materials and Design 31 (2010) 1993–2000
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Role of micro/nano fillers on mechanical and tribological properties of polyamide66/polypropylene composites B. Suresha a,*, B.N. Ravi Kumar b, M. Venkataramareddy b, T. Jayaraju a a b
Department of Mechanical Engineering, The National Institute of Engineering, Mysore-570 008, Karnataka, India Department of Mechanical Engineering, Bangalore Institute of Technology, Bangalore-560 004, Karnataka, India
a r t i c l e
i n f o
Article history: Received 12 August 2009 Accepted 16 October 2009 Available online 21 October 2009 Keywords: Micro/nano filler filled polyamide66/ polypropylne composites Mechanical properties Sliding wear Wear rate Scanning electron microscopy
a b s t r a c t The objectives of this research article is to evaluate the mechanical and tribological properties of polyamide66/polypropylene (PA66/PP) blend, graphite (Gr) filled PA66/PP, nanoclay (NC) filled PA66/PP and NC plus short carbon fiber (NC + SCF) filled PA66/PP composites. All composites were fabricated using a twin screw extruder followed by injection molding. The mechanical properties such as tensile, flexure, and impact strengths were investigated in accordance with ASTM standards. The friction and sliding wear behaviour was studied under dry sliding conditions against hard steel on a pin-on-disc apparatus. Scanning electron micrographs were used to analyze the fracture morphologies. From the experimental investigation, it was found that the presence of NC and SCF fillers improved the hardness of PA66/PP blend. Further, the study reveals that the tensile and flexural strength of NC + SCF filled PA66/PP was higher than that of PA66/PP blend. Inclusion of micro and nanofillers reduced the wear rate of PA66/PP blend. The wear loss of the composites increased with increasing sliding velocity. The lowest wear rate was observed for the blend with nanoclay and SCF fillers. The wear rates of the blends with micro/nanofillers vary from 30–81% and lower than that of PA66/PP blend. The wear resistance of the PA66/PP composites was found to be related to the stability of the transfer film on the counterface. The results have been supplemented with scanning electron micrographs to help understand the possible wear mechanisms. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Composite materials provide an opportunity to combine different properties and design materials for applications requiring multiple functionalities. Polymeric matrices reinforced with hard and non dissipative fillers can possess high stiffness and damping, which is ideal for structural properties. Over the past decades, injection molded thermoplastic composites have been increasingly used for numerous mechanical and tribological purposes such as seals, gears and bearings. These materials are light in weight and are better alternatives to metallic components [1,2]. The feature that makes polymer composites so promising in industrial applications is the possibility of tailoring their properties with functional fillers. It has been found that short fiber reinforcements can generally improve mechanical properties of the polymer composites [3,4]. Filler-reinforced thermoplastic composites can improve the stiffness, decrease thermal expansion, improve long-term mechanical performance and reduce costs [5]. Many materials, including mineral and glass fillers, glass beads, carbon black, and wood flour, are currently used as fillers in thermoplastic composites [6,7].
* Corresponding author. Tel.: +91 821 2480475; fax: +91 821 485802. E-mail address: [email protected] (B. Suresha). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.10.031
Polyamide66 (PA66) is a semi crystalline thermoplastic polymer used for numerous engineering applications. Many investigations on the mechanical and tribological properties of PA66 and its composites have been carried out. Qin et al. [8] found that PA66/clay nanocomposite exhibits better thermal stability and flame retardancy compared to pure PA66. Liu et al. [9] demonstrated that the introduction of silicate layers into the matrix induced the appearance of the (c) phase, changed the (a) crystalline phase and also reduced the Brill transition temperature because of the interaction between polyamide and surfaces of silicate layers. Shen et al. [10,11] studied the clay loadings and strain rate on the nanoindentation behaviour of PA66. Integration of various functional fillers is an important route in the design of load bearing and wear resistant polymer composites. Unal et al. [12] studied the influence of talc and kaolin fillers on the mechanical properties of PA6. Fillers, either individually or in mixed form by various weight ratios (10–30 wt.%) were added to PA6. The results showed that the tensile strength and modulus of PA6 composite increased with the increase in filler content, whereas the impact strength and elongation at break decreased with the increase in filler content. Addition of 10–15 wt.% of filler was found to be optimal in PA6. Hattotuwa et al. [13] studied the unmodified and ground talc and rice husk fillers compounded with polypropylene (PP) separately. The mechanical properties of the
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composites with reference to filler type and its loading were investigated. In terms of mechanical properties, Young’s modulus and flexural modulus increased, whereas yield strength and elongation at break decreased with the increase in filler loading for both types of composite. Polymeric nanocomposites consisting of inorganic nano-sized building blocks and organic polymers represent a new class of materials owing to their unique properties resulting from the nano-scale microstructure. Mechanical properties of these composites can be improved significantly at rather low filler content if the ultrafine phase dimensions of the nanoparticles are maintained [14]. Until now, however, there has been little investigation on the development of polymer composites by incorporating nano particles with conventional fillers. Hence, it is worthwhile exploring further, the load carrying capacity of such composite systems compounded with micro and nano particulate fillers. Before this potential can be fully realized, a good understanding of the role of fillers, especially addition of nano particles, in modifying the mechanical behaviour of polymer composites is essential. Such understanding will facilitate the formulation of optimal criteria for the selection of materials subjected to different mechanical loadings. The mechanical properties of common polymers when compared to metals are not very good. This has prompted to attempt cast particulate filled polymer composites. Considerable attention has thus been paid in the last 30 years to study the tribological properties of polymer composites. Reviews of such works may be found in articles by Briscoe and Tweedale [15], Sinha and Biswas [16], Zum Gahr [17], Friedrich et al. [18], Suh [19], Bijwe et al. [20] and Suresha et al. [21]. Some of the fillers that are effective in reducing friction and wear are MoS2, CuO, CuS, and Al2O3. The use of graphite as a filler material is known to improve the mechanical and tribological properties of polymer matrix composites [22]. Kishore et al. [23] analyzed the influence of sliding speed and load on the friction and wear behaviour of glass-reinforced polymer composites filled with either rubber or Al2O3 oxide particles; they reported that the wear loss increased with increasing load/speed. Most of the previous findings were based on micro sized fillers or short fibers. Polypropylene exhibits many beneficial properties such as low density, relative high thermal stability, and resistance to chemical attack, easy processing and recyclability. The property that accounts for the popularity of polyamide is high crystalline melting point good resistance to hydrocarbons, high strength, and ease of processing and fabrication. Mixing two polymers usually leads to immiscible blends, characterized by a coarse, metastable morphology, and poor adhesion between the phases. For improved performance the immiscible blends usually need compatibilization. Hence in this research work, the influence of nanoclay, graphite, and short carbon fiber on the mechanical and tribological properties of PA66/PP composites with maleic anhydride polypropylene as compatibilizer was investigated.
Table 1 Data on polymer, particulates and compatibilizing agent. Polymer/filler designation
Melting point (°C)
Density (g/cc)
Source
Polyamide66 (PA66) Polypropylene (PP) Maleic anhydride grafted PP (MAgPP) Nanoclay (NC) Short carbon fiber (SCF) Graphite (Gr)
263 168 133
1.14 0.90 0.96
DuPont Co. Ltd. IPCL, India DuPont Co. Ltd.
>1000 >1000 >1000
2.35 1.74 2.12
Sigma–Aldrich Inc. Fibraplex Corp Celina TN Graphite India Ltd.
Table 2 Sample designation and composition of ingredients used in the study. Sl no.
Sample (designation)
1. 2. 3. 4. 5. 6. 7. 8.
Polyamide66 Polypropylene PA66/PP 2%NC-PA66/PP 3%NC- PA66/PP 2.5%Gr-PA66/PP 5%Gr-PA66/PP 2%NC + 10%SCF-PA66/PP
Composition by wt.% PP
PA66
Particulate filler
– 100 50 48 47 47.5 45 37.5
100 – 50 50 50 50 50 50
– – – 2 (NC) 3 (NC) 2.5 (Gr) 5 (Gr) 2 + 10 (NC + SCF)
2.2. Compounding Before compounding, the polymer granules and fillers were dried at 80 °C for 10 h in an air circulated oven and then dry mixed with polyamide66 and other additives. Composition shown in Table 2 was mixed and extrudated in a co-rotating twin extruder. The L/D ratio of the screw is 40:1. Mixing speed of 60 rpm was maintained for all the compositions. The extrudate from the die were quenched in a tank at 20–30 °C and then palletized. The temperatures from the feed zone to the die of the extruder were 205, 235, 245, 255 and 265 °C, respectively. The extrudate of the composition was palletized in palletizing machine. 2.3. Injection molding The granules of the extrudates were pre dried in an air circulated oven at 80 °C for 10 h and injection molded in a microprocessor based injection molding machine fitted with a master mould containing the cavity for tensile strength, flexural and impact specimens. After its ejection from the mould, specimens were cooled in ice-water. The temperatures maintained in three zones of the barrel were 200 °C, 235 °C and 260 °C, respectively. The details of the composites fabricated for the present investigation are given in Table 2. 2.4. Density and hardness measurement
2. Materials and methods 2.1. Material used Polymer alloy of polyamide66 and polypropylene, and particulates (nanoclay, graphite and short carbon fiber) filled PA66/PP composites were prepared for this study with compatibilizer. The polymer alloy produced consists of 50% by weight. Maleic anhydride polypropylene (MAgPP) as compatibilizing agent was used in this study. The amount of compatibilizer added was 1 wt.% based on previous literature and this compatibilizer proportion was high enough for interaction with PA66/PP interface. The sources and characteristics of these materials are listed in Table 1.
Densities of the composites were determined by using a high precision electronic balance (Mettler Toledo, Model AX 205) using Archimede’s principle. Shore hardness of the samples was measured as per ASTM D2240, by using a Hiroshima make Hardness Tester (Durometer). Six readings at different locations were noted and average value is reported. 2.5. Mechanical properties The tensile and flexural measurements were carried out using Universal tensile testing machine (JJ Lloyd, London, United Kingdom, capacity 1–20 KN), according to ASTM D638 and ASTM
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D790 respectively. The tensile and flexure tests were performed at crosshead speed of 20 mm/min (quasi-static) and 2.4 mm/min respectively. Five samples were tested for each composition of the composites. Izod impact test were carried out using an Avery Denison impact tester (ASTM D256–92). A 2.75 J energy hammer was used and the striking velocity was 3.46 m/s. For Izod impact test specimens, the notch was cut using a motorized notch-cutting machine. 2.6. Friction and sliding wear testing A pin-on-disc setup was used for sliding wear experiments. The surface (5 mm 5 mm), glued to a pin with a diameter of 6 mm and a length of 22 mm, came in contact with a hardened disc with a hardness of 62 HRc. It was made of En 32 steel with a diameter of 160 mm, a thickness of 8 mm, and a surface roughness of 0.46 lm. The test was conducted on a track with a diameter of 115 mm by selection of the test duration, load, and velocity in accordance with ASTM G-99. Before testing, the test samples were polished against 600-grade SiC paper to ensure proper contact with the countersurface. The surfaces of both the sample and the disc were cleaned with a soft paper soaked in acetone and were thoroughly dried before the test. The pin assembly was initially weighed to an accuracy of 0.0001 g in a digital electronic balance (Mettler Toledo). The test was carried out by application of different loads (20 and 80 N) and sliding velocities (3–6 m/s) at a constant sliding distance of 6000 m. The difference between the initial and final weights was a measure of slide wear loss. For each condition, at least three tests were performed, and the mean value of weight loss was reported. A 20-kg load cell was fixed tangential to the lever arm, through which the friction force was measured. 2.7. Scanning electron microscopy After tension test, the fractured samples were examined using a scanning electron microscope (JSM 840A model and JEOL make). Before the examinations, a thin gold film was deposited on the tensile fractured surface. 3. Results and discussion 3.1. Density and hardness Table 3 shows the measured densities of PA66/PP blend, NCPA66/PP, Gr-PA66/PP and NC + SCF-PA66/PP composites. The densities of all nanocomposites are higher than the density of PA66/ PP blend. The density of NC + SCF filled PA66/PP composite is 1.256 which is highest when compared to other composites. This is because the fillers, nanoclay and short carbon fibers have higher density than that of PA66/PP. By using Duro-hardness tester, the hardness of the composites is measured; the values recorded are
given in Table 3. The hardness of PA66/PP blend increased with inclusion of particulate fillers. From Table 3, it can be seen that the nanoclay/short carbon fiber greatly increased the hardness of PA66/PP blend, which can be attributed to the higher hardness and more uniform dispersion of nanoclay, and short carbon fiber. The higher hardness is exhibited by the NC + SCF filled PA66/PP compared to other nanocomposites. The hardness of NC + SCF filled PA66/PP composite is 75, which is highest among all the composites tested. The improvement in hardness with incorporation of filler can be explained as follows: under the action of a compressive force, the thermoplastic matrix phase and the solid filler phase will be pressed together, touch each other and offer resistance. Thus the interface can transfer load more effectively although the interfacial bond may be poor. This results in enhancement of hardness of filled composites. 3.2. Tensile behaviour Quasi-static tension tests were performed on five samples of each type of composite as well as on unfilled PA66/PP blend. Typical engineering stress–strain curves are shown in Fig. 1. These curves illustrate evidence of plastic deformation prior to failure, which occurred in the case of unfilled, NC filled and graphite filled PA66/PP composites. Properties of micro/nano filler filled polymer composites depend on the characteristics of fillers used. Therefore, the mechanical properties of composites are dependent on the characteristics of matrix, filler as well as interface phase. In the present work, all composites exhibited higher tensile strengths than neat PP, and only the nanoclay plus short carbon fiber filled PA66/PP exhibited significant strain hardening. Tensile properties derived from the curves are given in Table 3. Due to addition of PA66 to PP, the tensile strength increases (by 64%). Further, the results illustrates that 2%NC + 10%SCF-PA66/PP exhibits a tensile strength (50.00 ± 2.34 MPa) that is almost 63% higher than that of PA66/PP (30.65 ± 2.05 MPa), where as the tensile strength of PA66/PP containing nanoclay (22.50 ± 1.86 MPa) is also less than that of PA66/PP blend. From Table 3, it can also be seen that the nanoclay/short carbon fiber greatly increased the tensile strength of PA66/PP composite. However, the tensile strength improvement occurs sacrificing the ductility of the polymer blend similar to any engineering material. Micro/nano filler inclusion though improves the mechanical strength imparts brittleness of the composites. Addition of graphite as filler in PA66/PP resulted in slight increase in the tensile strength and percentage elongation. Nanoclay inclusion increased the tensile strength of PA66/PP blends. It is believed that the tensile strength of clay filled PA66/ PP composites depends on several factors such as dispersion of 60
PA66 PP
50
PA66/PP
Table 3 Mechanical properties of composites. Sl no.
Sample designation
Tensile strength (MPa)
Strain
9. 10. 11. 12. 13. 14. 15. 16.
PA66 PP PA66/PP 2%NC-PA66/PP 3%NC- PA66/PP 2.5%Gr-PA66/PP 5%Gr-PA66/PP 2%NC + 10%SCFPA66/PP
48.94 18.71 30.65 22 21.7 30.65 30.9 49
0.15 0.21 0.13 0.08 0.06 0.11 0.10 0.07
Hardness (shore-D)
Density (g/cc)
80 41 64 71 74 66 71 75
1.09 0.9034 0.9072 0.9705 1.0282 0.9843 0.9904 1.256
Stress (MPa)
2%NC-PA66/PP
40
3%NC-PA66/PP 2.5%Gr-PA66/PP
30
5%Gr-PA66/PP 2%NC+10%SCFPA66/PP
20 10 0
0
0.05
0.1
0.15
0.2
0.25
Strain Fig. 1. Engineering stress–strain curves generated from tensile tests for the composites.
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the nanoclay in the skin and core layers, interaction of PA66/PP with nanoclay, compatibility of PA66 with PP, interaction of PP with nanoclay and filler–filler interaction of the nanoclay [14]. Increase in tensile strength of PA66/PP blend with addition of nanoclay and short carbon fiber may be due to better interaction between them and uniform distribution of nanoclay and SCF. The effect of different fillers on the elongation at break of PA66/PP composites are also given in Table 3. The elongation at break decreased drastically with the incorporation of different fillers. The brittleness of these composites can be associated with the disappearance of plastic deformation of polymer matrix. In the experimental range, the best mechanical properties were obtained with the composite having 2 wt.% of nanoclay plus 10 wt.% of short carbon fiber with compatibilizer. PA66/PP blend showed lower tensile strength and higher strain. The lower strain is observed in NC + SCF filled PA66/PP composite as compared to other composites. It could be due to addition of nanoclay and short carbon fiber in PA66/PP, reducing the interfacial distance between the matrix and filler. Additionally, the nanoclay particles may provide obstacles to cracks formed in SCF filled PA66/PP, resulting in less convenient crack path and therefore more difficult to fracture. 3.3. Flexural behaviour Flexural properties are of great importance for any structural element. Composite materials used in structures are prone to fail in bending and therefore the development of new composites with improved flexural characteristics is essential. Table 4 lists the flexural strength and modulus of particulate filled PA66/PP composites. It is observed that the flexural strength and modulus increased for NC + SCF filled PA66/PP and decreased for NC and graphite filled systems. It is interesting to note that addition of NC alone lowers the flexural strength drastically. However, in the case of Gr-PA66/PP, the increase in flexural strength is marginal. Among the three fillers selected in the present study, the inclusion of SCF and graphite causes maximum increase in strength of the composite. It could be due to good chemical reaction at the interface between the filler particles and matrix results in better load transfer. 3.4. Impact behaviour While the influence of the filler particles on the tensile strength is rather limited, the influence on the impact strength is significant. The impact strength of different composites recorded during the test are also given in Table 4. It shows that unfilled PA66/PP and graphite filled PA66/PP composite exhibited higher impact strength than that of NC + SCF filled ones. The best performance was achieved with 5 wt.% graphite addition in PA66/PP blend. The addition of fine graphite particles led to 102% higher impact strength than the neat PP. This suggests that for graphite filled
composite subject to impact loading, the interfacial regions are able to resist crack propagation more effectively than the polymer blend. Similar behaviour has also been reported in rubber filled polymer composites. Higher filler loading, usually reduces the impact strength and may be attributed to the agglomeration of filler particles [14]. However, because of lower toughness, the addition of SCF and nanoclay decreased the impact strength of the blend indicating that the compatibility between the polymer matrix and the particulates is poor. The results of impact tests in the present study reveal that graphite can be a promising filler material that would enhance the impact characteristics of polymer composites. Although it is believed that the mechanical properties of a composite are fiber reinforcement dependent, inclusion of fillers in the form of particulates and SCF in PA66/PP influenced the mechanical properties. The results show that improvement in impact strength was achieved with graphite as filler in PA66/PP blend. 3.5. Fractography Very little has been published on the fracture surface morphology of filled polymer nanocomposites by using scanning electron microscopy. It is also known that SEM is not a suitable technique to examine the morphology of polymer nanocomposites. However, in the present work, SEM analysis was used to investigate the effect of the nanoclay, graphite and SCF on the tensile fractured surface morphologies of PA66/PP and their composites. Figs. 2–4 show the SEM pictures of the tensile fractured surface of composite samples. Fig. 2 shows the photomicrograph of the tensile fractured surface of unfilled PA66/PP blend. Big PP particles formed are shown by arrows. They induce some cavitations in PA66 because of which a fibrillar structure appears. The incompatibility of PA66 and PP blends could be observed through PA66 fibril structure and the PP particles pulled out from the PA66 matrix. Similar observations were also found in the literature [24]. Further, the photomicrograph showed brittle fracture of the PA66/PP and pullout of the PP particles. It may be the primary mechanism for fracture. Also, irregular shaped, large PP particles dispersed in the PA66 matrix can be seen. These particles easily detach from the PA66 matrix due to poor interfacial adhesion. The photomicrograph of the tensile fractured surface of 2 wt.% nanoclay filled PA66/PP composite is shown in Fig. 3. The fibrillated morphology observed in PA66/PP blend reduced in this case. The big cavities as shown in Fig. 2 are substantially reduced owing to the presence of MAgPP compatibilizer. However, the presence of
Table 4 Impact strength, flexural stress and flexural modulus values of the composites. Sl no.
Sample designation
Flexural strength (N/mm2)
Flexural modulus (MPa)
Impact strength (J/m)
1. 2. 3. 4. 5. 6.
PA66 PP PA66/PP 2%NC-PA66/PP 3%NC-PA66/PP 2.5%Gr-PA66/ PP 5%Gr-PA66/PP 2%NC + 10%SCF PA66/PP
84.33 30.71 32.06 20.3 19.3 27.4
2654 910 668.80 567.83 567.86 708.28
85 27.82 56 25 18.75 56.25
24.4 51.4
719.25 1009.57
56.25 25
7. 8.
Fig. 2. Scanning electron micrograph of tensile fracture of unfilled PA66/PP blend.
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Table 5 Coefficient of friction of PA66/PP composites under all experimental conditions. Load (N)/composite
Velocity (m/s) 3
PA66/PP 2%NC-PA66/PP SCF + NC-PA66/PP 2.5%Gr-PA66/PP
Fig. 3. Scanning electron micrograph of tensile fracture of 2%NC filled PA66/PP nanocomposite.
4
5
6
20
80
20
80
20
80
20
80
0.54 0.59 0.30 0.33
0.50 0.56 0.25 0.31
0.52 0.58 0.25 0.30
0.50 0.56 0.21 0.27
0.51 0.54 0.24 0.29
0.47 0.52 0.20 0.26
0.50 0.53 0.23 0.25
0.45 0.51 0.20 0.21
6000 m as a function of sliding velocity at different applied loads of 20 and 80 N. This is obtained by dividing the frictional force by the applied normal load and are listed in Table 5. As the applied normal load increases, the coefficient of friction decreases and an increase in sliding velocity results in faster sliding on the counterface, which causes less frictional force and hence a lower coefficient of friction. Differing trends in the values of coefficient of friction were observed for PA66/PP, 2%NC-PA66/PP, 2.5%Gr-PA66/ PP and NC + SCF-PA66/PP composite systems. The variation in wear loss of unfilled and particulate filled PA66/PP composites under different sliding velocities and loads (20 and 80 N) are shown in Figs. 5 and 6 respectively. The experimental results reveal that the wear loss increases linearly with sliding velocity at a constant load. The trend in the wear loss with sliding velocity is the same for all samples irrespective of the load employed. Another interesting feature observed is that the wear loss increases gradually with increase in sliding velocity for
Fig. 4. Scanning electron micrograph of tensile fracture of 2%NC + 10%SCF filled PA66/PP nanocomposite.
PP particles that were detached from the PA66 matrix is obvious from the fracture surface. The reduced fibrillated structure is a direct manifestation of the missing plastic deformation of PA66 matrix due to the incorporation of nanoclay. Fig. 4 shows the photomicrograph of 2 wt.% NC and 10 wt.% SCF filled PA66/PP composite. PP particles are still detached from the PA66 matrix. Further, microcracks can be seen from the fractured surface. Neither a fibrillated morphology nor PP particles detached from the PA66 matrix could be observed. The number of available reinforcing elements is increased due to uniform dispersion of NC and SCF and this in turn enhanced the load carrying capacity. The improvement in mechanical properties is attributed to better interfacial adhesion between the fillers and blend. Further, short carbon fiber may act as a bridge between the phases and thus enhance the load carrying capacity of the blend. This behaviour is reflected from the photomicrograph shown in Fig. 4. The presence of microcracking caused by stress concentration sites (i.e., agglomerated nanoclay and SCF particulates) provides better evidence for the brittleness of the PA66/PP composite.
Fig. 5. Wear loss versus sliding velocity at a load of 20 N and a sliding distance of 6000 m.
3.6. Friction and sliding wear results The friction coefficient of unfilled and particulate filled PA66/PP composites are obtained under the constant sliding distance of
Fig. 6. Wear loss versus sliding velocity at a load of 80 N and a sliding distance of 6000 m.
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2%NC-PA66/PP and NC + SCF-PA66/PP, whereas in the unfilled and 2.5%Gr-PA66/PP, a higher gradient is seen. The wear loss increases with increasing load for all samples and shows an upward trend indicating greater wear loss at high load (Fig. 6). The primary reasons for adding fillers or reinforcing fibers to polymers is to improve their mechanical properties, but the effects on wear rate are not invariably beneficial. Suresha et al. [22–27] investigated the wear behaviour of particulate filled glass/carbon fabric reinforced epoxy composites. The above investigations were concentrated on sliding and three-body/two-body abrasive wear studies. The silica sand was used as dry and loose abrasives in three-body wear tests. In abrasive wear situations, the wear rate depends on the experimental test parameters such as load and abrading distance. The fillers such as cenosphere and graphite were observed to be detrimental to abrasive wear performance. Graphite filler in carbon epoxy composite provides better wear resistance under dry sliding wear tests. In the present study, the highest wear resistance is for NC + SCF filled PA66/PP composites. In previous studies, the effect of particular fillers on mechanical and three-body abrasive wear behaviour of polyamide66/polypropylene nanocomposites has been investigated [28]. Here abrasive wear tests were performed at different loads/abrading distances. It was noticed that particulate fillers especially short carbon fiber improved the mechanical properties and the wear volume of unfilled PA66/PP blend with respect to (a) NC-PA66/PP and (b) NC + SCF-PA66/PP composites respectively show 142% and 216% decrease. The reduction in wear volume of unfilled PA66/PP is related to an inclusion of MAgPP as compatiblizer results in lower interfacial surface tension, finer and stabilized phase morphologies. As shown in Figs. 7 and 8, the wear rate of PA66/PP compos-
Fig. 7. Wear rate of particulate filled PA66/PP nanocomposites at a load of 20 N and a sliding distance of 6000 m.
Fig. 8. Wear rate of particulate filled PA66/PP nanocomposites at a load of 80 N and a sliding distance of 6000 m.
ites with the addition of particulate fillers was clearly lower than that of the blend without particulate fillers. The wear rate of composites increases with increasing sliding velocity and load. The wear of the composites appears to be strongly influenced by the stability of the transfer film on the counterface. The addition of short carbon fibers remarkably enhanced the tribological performance of NC filled PA66/PP. It can be seen that the coefficient of friction was reduced with SCF and the wear rate of the composite was doubly decreased. The addition of short carbon fibers to the nanoclay filled PA66/PP arrests stick–slip and induces large plastic flow at the surface. This appears to stabilize the film, which now interacts with the counterface film in a low adhesion regime resulting in less wear. 3.7. Worn surface morphology The worn surface of the composites was observed by SEM examination to search for a correlation of worn surface and slide wear loss with increasing sliding velocities/loads. Figs. 9–12 show morphology of the worn surface of the PA66/PP blend, 2%NC-PA66/ PP, NC + SCF-PA66/PP and 2.5%Gr-PA66/PP composites at 6000 m, 80 N load at a velocity of 6 m/s test conditions. The photomicrograph of the worn surface of PA66/PP blend is as shown in Fig. 9. Due to mechanical action and the accumulated frictional heat the molecular structure of PA66/PP blend has been partially destroyed. The interfacial bonding strength is weaker for the blend thereby, producing severe plastic deformation. PP has straight molecular chain and relatively lower softening point. The poor adhesion between PA66 and PP results in higher wear loss at higher load and velocity. The photomicrograph of the worn surface of 2.5%NC-PA66/PP showed in Fig. 10. It reveals a network of microcracks and few furrows in the sliding direction due to incorporation of nanoclay. Elastic and plastic deformations on the surface of PA66/PP become the main cause for the wear under the effect of nanoclay inclusion. The repeated deformation causes cracking on the surface or subsurface of the composite. The photomicrograph of the worn surface of NC + SCF-PA66/PP composite is as shown in Fig. 11. The wear rate was reduced by more than twice in magnitude when 10 wt.% of SCF as added. The beneficial effect as a result of SCF reinforcement in mainly due to a reduced ability of ploughing, tearing and other non-adhesive components of wear [29]. At the beginning, the surface is cut by the asperities of the counterpart. Micro-furrows are generated on the surface of the sample, and the transfer film is formed on the surface of the counterpart. The photomicrograph of the worn surface of 2.5% Gr-PA66/PP composite is shown in Fig. 12. It reveals
Fig. 9. Scanning electron micrograph of PA66/PP blend at 80 N, 6000 m and 6 m/s.
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4. Conclusions
Fig. 10. Scanning electron micrograph of 2%NC-PA66/PP at 80 N, 6000 m and 6 m/s.
Fig. 11. Scanning electron micrograph of 10%SCF + 2%NC-PA66/PP blend at 80 N, 6000 m and 6 m/s.
The study shows the successful fabrication of unfilled and filled PA66/PP composites. The structural behaviour of PA66/PP blend and their composites was evaluated using static and quasi- static mechanical tests. The material with 2%NC + 10%SCF-PA66/PP composite showed the most superior mechanical properties with a tensile strength of 50 MPa, hardness 75 (Shore-D), flexural strength of 52 MPa and flexural modulus of 1010 MPa. Further, NC + SCF exhibit strain hardening in addition to increased strength. The superior mechanical properties and the strain hardening exhibited by NC + SCF filled PA66/PP composite can be explained by the increased polymer network density caused by physical entanglements from the dispersion of nanoclay and short carbon fibers in the blend. A significant improvement was found in the toughness by performing impact tests. An enhancement of 102% was measured with fine graphite particles in PA66/PP. In the material with PA66/PP blend, the PP phase was present in the form of large particles in PA66 matrix. The interfacial adhesion between PA66 and PP was very poor. The hardness, density, tensile and flexural properties of the composite are also greatly influenced by the type and content of micro or nano fillers. Hence while fabricating a composite of specific requirements, there is a need for the choice of appropriate filler material and for optimizing its content in the composite. NC + SCF-PA66/PP composite with compatibilizer showed better resistance to dry sliding wear compared to unfilled and filled PA66/PP composites. Combination of short carbon fiber and nanoclay is very effective in reducing the coefficient of friction in sliding against steel counterface. SEM observations throw further light on features such as microcracks, pull out of PP particles, debonding of filler from the matrix and transfer film formation. The wear resistance of the micro/nano filler filled PA66/PP composites can be utilized to systematically design a composite material with a good wear performance. In conclusion, an optimum wear resistance can be expected from a composite consisting of PA66/PP blend and NC + SCF filler with MagPP compatibilizer. These experimental results indicate that in the context of application as a bearing material (journal bearings and antifriction bearings), the performance of PA66/PP blend and NC + SCF filler with MagPP compatibilizer is good and possibly superior to unfilled PA66.PP composites.
Acknowledgments We would like to thank N. Govinda Raju, Chief Executive Officer, Magnum Engineers, Bangalore, for providing the testing facilities. The authors are thankful to the Management and Principal Dr. M.S. Shiva Kumar, The National Institute of Engineering, Mysore, India, for their encouragement. The authors are also thankful to M. Ganesh, Managing Director, GLS Polymers Pvt. Ltd., Bangalore, for extending the fabrication facility for this research work. The authors also acknowledge the contributions of Dr. P.R. Sadananda Rao in preparation of this manuscript.
References Fig. 12. Scanning electron micrograph of 2.5%Gr-PA66/PP blend at 80 N, 6000 m and 6 m/s.
that due to heat generated by friction; destroy the molecular structure of Gr-PA66/PP composite. Fig. 12 also shows a smooth film with microcracks at the centre and the ploughed zone marked by extensive plastic flow and deformation.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Role of micro/nano fillers on mechanical and tribological properties of polyamide66/polypropylene composites B. Suresha a,*, B.N. Ravi Kumar b, M. Venkataramareddy b, T. Jayaraju a a b
Department of Mechanical Engineering, The National Institute of Engineering, Mysore-570 008, Karnataka, India Department of Mechanical Engineering, Bangalore Institute of Technology, Bangalore-560 004, Karnataka, India
a r t i c l e
i n f o
Article history: Received 12 August 2009 Accepted 16 October 2009 Available online 21 October 2009 Keywords: Micro/nano filler filled polyamide66/ polypropylne composites Mechanical properties Sliding wear Wear rate Scanning electron microscopy
a b s t r a c t The objectives of this research article is to evaluate the mechanical and tribological properties of polyamide66/polypropylene (PA66/PP) blend, graphite (Gr) filled PA66/PP, nanoclay (NC) filled PA66/PP and NC plus short carbon fiber (NC + SCF) filled PA66/PP composites. All composites were fabricated using a twin screw extruder followed by injection molding. The mechanical properties such as tensile, flexure, and impact strengths were investigated in accordance with ASTM standards. The friction and sliding wear behaviour was studied under dry sliding conditions against hard steel on a pin-on-disc apparatus. Scanning electron micrographs were used to analyze the fracture morphologies. From the experimental investigation, it was found that the presence of NC and SCF fillers improved the hardness of PA66/PP blend. Further, the study reveals that the tensile and flexural strength of NC + SCF filled PA66/PP was higher than that of PA66/PP blend. Inclusion of micro and nanofillers reduced the wear rate of PA66/PP blend. The wear loss of the composites increased with increasing sliding velocity. The lowest wear rate was observed for the blend with nanoclay and SCF fillers. The wear rates of the blends with micro/nanofillers vary from 30–81% and lower than that of PA66/PP blend. The wear resistance of the PA66/PP composites was found to be related to the stability of the transfer film on the counterface. The results have been supplemented with scanning electron micrographs to help understand the possible wear mechanisms. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Composite materials provide an opportunity to combine different properties and design materials for applications requiring multiple functionalities. Polymeric matrices reinforced with hard and non dissipative fillers can possess high stiffness and damping, which is ideal for structural properties. Over the past decades, injection molded thermoplastic composites have been increasingly used for numerous mechanical and tribological purposes such as seals, gears and bearings. These materials are light in weight and are better alternatives to metallic components [1,2]. The feature that makes polymer composites so promising in industrial applications is the possibility of tailoring their properties with functional fillers. It has been found that short fiber reinforcements can generally improve mechanical properties of the polymer composites [3,4]. Filler-reinforced thermoplastic composites can improve the stiffness, decrease thermal expansion, improve long-term mechanical performance and reduce costs [5]. Many materials, including mineral and glass fillers, glass beads, carbon black, and wood flour, are currently used as fillers in thermoplastic composites [6,7].
* Corresponding author. Tel.: +91 821 2480475; fax: +91 821 485802. E-mail address: [email protected] (B. Suresha). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.10.031
Polyamide66 (PA66) is a semi crystalline thermoplastic polymer used for numerous engineering applications. Many investigations on the mechanical and tribological properties of PA66 and its composites have been carried out. Qin et al. [8] found that PA66/clay nanocomposite exhibits better thermal stability and flame retardancy compared to pure PA66. Liu et al. [9] demonstrated that the introduction of silicate layers into the matrix induced the appearance of the (c) phase, changed the (a) crystalline phase and also reduced the Brill transition temperature because of the interaction between polyamide and surfaces of silicate layers. Shen et al. [10,11] studied the clay loadings and strain rate on the nanoindentation behaviour of PA66. Integration of various functional fillers is an important route in the design of load bearing and wear resistant polymer composites. Unal et al. [12] studied the influence of talc and kaolin fillers on the mechanical properties of PA6. Fillers, either individually or in mixed form by various weight ratios (10–30 wt.%) were added to PA6. The results showed that the tensile strength and modulus of PA6 composite increased with the increase in filler content, whereas the impact strength and elongation at break decreased with the increase in filler content. Addition of 10–15 wt.% of filler was found to be optimal in PA6. Hattotuwa et al. [13] studied the unmodified and ground talc and rice husk fillers compounded with polypropylene (PP) separately. The mechanical properties of the
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composites with reference to filler type and its loading were investigated. In terms of mechanical properties, Young’s modulus and flexural modulus increased, whereas yield strength and elongation at break decreased with the increase in filler loading for both types of composite. Polymeric nanocomposites consisting of inorganic nano-sized building blocks and organic polymers represent a new class of materials owing to their unique properties resulting from the nano-scale microstructure. Mechanical properties of these composites can be improved significantly at rather low filler content if the ultrafine phase dimensions of the nanoparticles are maintained [14]. Until now, however, there has been little investigation on the development of polymer composites by incorporating nano particles with conventional fillers. Hence, it is worthwhile exploring further, the load carrying capacity of such composite systems compounded with micro and nano particulate fillers. Before this potential can be fully realized, a good understanding of the role of fillers, especially addition of nano particles, in modifying the mechanical behaviour of polymer composites is essential. Such understanding will facilitate the formulation of optimal criteria for the selection of materials subjected to different mechanical loadings. The mechanical properties of common polymers when compared to metals are not very good. This has prompted to attempt cast particulate filled polymer composites. Considerable attention has thus been paid in the last 30 years to study the tribological properties of polymer composites. Reviews of such works may be found in articles by Briscoe and Tweedale [15], Sinha and Biswas [16], Zum Gahr [17], Friedrich et al. [18], Suh [19], Bijwe et al. [20] and Suresha et al. [21]. Some of the fillers that are effective in reducing friction and wear are MoS2, CuO, CuS, and Al2O3. The use of graphite as a filler material is known to improve the mechanical and tribological properties of polymer matrix composites [22]. Kishore et al. [23] analyzed the influence of sliding speed and load on the friction and wear behaviour of glass-reinforced polymer composites filled with either rubber or Al2O3 oxide particles; they reported that the wear loss increased with increasing load/speed. Most of the previous findings were based on micro sized fillers or short fibers. Polypropylene exhibits many beneficial properties such as low density, relative high thermal stability, and resistance to chemical attack, easy processing and recyclability. The property that accounts for the popularity of polyamide is high crystalline melting point good resistance to hydrocarbons, high strength, and ease of processing and fabrication. Mixing two polymers usually leads to immiscible blends, characterized by a coarse, metastable morphology, and poor adhesion between the phases. For improved performance the immiscible blends usually need compatibilization. Hence in this research work, the influence of nanoclay, graphite, and short carbon fiber on the mechanical and tribological properties of PA66/PP composites with maleic anhydride polypropylene as compatibilizer was investigated.
Table 1 Data on polymer, particulates and compatibilizing agent. Polymer/filler designation
Melting point (°C)
Density (g/cc)
Source
Polyamide66 (PA66) Polypropylene (PP) Maleic anhydride grafted PP (MAgPP) Nanoclay (NC) Short carbon fiber (SCF) Graphite (Gr)
263 168 133
1.14 0.90 0.96
DuPont Co. Ltd. IPCL, India DuPont Co. Ltd.
>1000 >1000 >1000
2.35 1.74 2.12
Sigma–Aldrich Inc. Fibraplex Corp Celina TN Graphite India Ltd.
Table 2 Sample designation and composition of ingredients used in the study. Sl no.
Sample (designation)
1. 2. 3. 4. 5. 6. 7. 8.
Polyamide66 Polypropylene PA66/PP 2%NC-PA66/PP 3%NC- PA66/PP 2.5%Gr-PA66/PP 5%Gr-PA66/PP 2%NC + 10%SCF-PA66/PP
Composition by wt.% PP
PA66
Particulate filler
– 100 50 48 47 47.5 45 37.5
100 – 50 50 50 50 50 50
– – – 2 (NC) 3 (NC) 2.5 (Gr) 5 (Gr) 2 + 10 (NC + SCF)
2.2. Compounding Before compounding, the polymer granules and fillers were dried at 80 °C for 10 h in an air circulated oven and then dry mixed with polyamide66 and other additives. Composition shown in Table 2 was mixed and extrudated in a co-rotating twin extruder. The L/D ratio of the screw is 40:1. Mixing speed of 60 rpm was maintained for all the compositions. The extrudate from the die were quenched in a tank at 20–30 °C and then palletized. The temperatures from the feed zone to the die of the extruder were 205, 235, 245, 255 and 265 °C, respectively. The extrudate of the composition was palletized in palletizing machine. 2.3. Injection molding The granules of the extrudates were pre dried in an air circulated oven at 80 °C for 10 h and injection molded in a microprocessor based injection molding machine fitted with a master mould containing the cavity for tensile strength, flexural and impact specimens. After its ejection from the mould, specimens were cooled in ice-water. The temperatures maintained in three zones of the barrel were 200 °C, 235 °C and 260 °C, respectively. The details of the composites fabricated for the present investigation are given in Table 2. 2.4. Density and hardness measurement
2. Materials and methods 2.1. Material used Polymer alloy of polyamide66 and polypropylene, and particulates (nanoclay, graphite and short carbon fiber) filled PA66/PP composites were prepared for this study with compatibilizer. The polymer alloy produced consists of 50% by weight. Maleic anhydride polypropylene (MAgPP) as compatibilizing agent was used in this study. The amount of compatibilizer added was 1 wt.% based on previous literature and this compatibilizer proportion was high enough for interaction with PA66/PP interface. The sources and characteristics of these materials are listed in Table 1.
Densities of the composites were determined by using a high precision electronic balance (Mettler Toledo, Model AX 205) using Archimede’s principle. Shore hardness of the samples was measured as per ASTM D2240, by using a Hiroshima make Hardness Tester (Durometer). Six readings at different locations were noted and average value is reported. 2.5. Mechanical properties The tensile and flexural measurements were carried out using Universal tensile testing machine (JJ Lloyd, London, United Kingdom, capacity 1–20 KN), according to ASTM D638 and ASTM
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D790 respectively. The tensile and flexure tests were performed at crosshead speed of 20 mm/min (quasi-static) and 2.4 mm/min respectively. Five samples were tested for each composition of the composites. Izod impact test were carried out using an Avery Denison impact tester (ASTM D256–92). A 2.75 J energy hammer was used and the striking velocity was 3.46 m/s. For Izod impact test specimens, the notch was cut using a motorized notch-cutting machine. 2.6. Friction and sliding wear testing A pin-on-disc setup was used for sliding wear experiments. The surface (5 mm 5 mm), glued to a pin with a diameter of 6 mm and a length of 22 mm, came in contact with a hardened disc with a hardness of 62 HRc. It was made of En 32 steel with a diameter of 160 mm, a thickness of 8 mm, and a surface roughness of 0.46 lm. The test was conducted on a track with a diameter of 115 mm by selection of the test duration, load, and velocity in accordance with ASTM G-99. Before testing, the test samples were polished against 600-grade SiC paper to ensure proper contact with the countersurface. The surfaces of both the sample and the disc were cleaned with a soft paper soaked in acetone and were thoroughly dried before the test. The pin assembly was initially weighed to an accuracy of 0.0001 g in a digital electronic balance (Mettler Toledo). The test was carried out by application of different loads (20 and 80 N) and sliding velocities (3–6 m/s) at a constant sliding distance of 6000 m. The difference between the initial and final weights was a measure of slide wear loss. For each condition, at least three tests were performed, and the mean value of weight loss was reported. A 20-kg load cell was fixed tangential to the lever arm, through which the friction force was measured. 2.7. Scanning electron microscopy After tension test, the fractured samples were examined using a scanning electron microscope (JSM 840A model and JEOL make). Before the examinations, a thin gold film was deposited on the tensile fractured surface. 3. Results and discussion 3.1. Density and hardness Table 3 shows the measured densities of PA66/PP blend, NCPA66/PP, Gr-PA66/PP and NC + SCF-PA66/PP composites. The densities of all nanocomposites are higher than the density of PA66/ PP blend. The density of NC + SCF filled PA66/PP composite is 1.256 which is highest when compared to other composites. This is because the fillers, nanoclay and short carbon fibers have higher density than that of PA66/PP. By using Duro-hardness tester, the hardness of the composites is measured; the values recorded are
given in Table 3. The hardness of PA66/PP blend increased with inclusion of particulate fillers. From Table 3, it can be seen that the nanoclay/short carbon fiber greatly increased the hardness of PA66/PP blend, which can be attributed to the higher hardness and more uniform dispersion of nanoclay, and short carbon fiber. The higher hardness is exhibited by the NC + SCF filled PA66/PP compared to other nanocomposites. The hardness of NC + SCF filled PA66/PP composite is 75, which is highest among all the composites tested. The improvement in hardness with incorporation of filler can be explained as follows: under the action of a compressive force, the thermoplastic matrix phase and the solid filler phase will be pressed together, touch each other and offer resistance. Thus the interface can transfer load more effectively although the interfacial bond may be poor. This results in enhancement of hardness of filled composites. 3.2. Tensile behaviour Quasi-static tension tests were performed on five samples of each type of composite as well as on unfilled PA66/PP blend. Typical engineering stress–strain curves are shown in Fig. 1. These curves illustrate evidence of plastic deformation prior to failure, which occurred in the case of unfilled, NC filled and graphite filled PA66/PP composites. Properties of micro/nano filler filled polymer composites depend on the characteristics of fillers used. Therefore, the mechanical properties of composites are dependent on the characteristics of matrix, filler as well as interface phase. In the present work, all composites exhibited higher tensile strengths than neat PP, and only the nanoclay plus short carbon fiber filled PA66/PP exhibited significant strain hardening. Tensile properties derived from the curves are given in Table 3. Due to addition of PA66 to PP, the tensile strength increases (by 64%). Further, the results illustrates that 2%NC + 10%SCF-PA66/PP exhibits a tensile strength (50.00 ± 2.34 MPa) that is almost 63% higher than that of PA66/PP (30.65 ± 2.05 MPa), where as the tensile strength of PA66/PP containing nanoclay (22.50 ± 1.86 MPa) is also less than that of PA66/PP blend. From Table 3, it can also be seen that the nanoclay/short carbon fiber greatly increased the tensile strength of PA66/PP composite. However, the tensile strength improvement occurs sacrificing the ductility of the polymer blend similar to any engineering material. Micro/nano filler inclusion though improves the mechanical strength imparts brittleness of the composites. Addition of graphite as filler in PA66/PP resulted in slight increase in the tensile strength and percentage elongation. Nanoclay inclusion increased the tensile strength of PA66/PP blends. It is believed that the tensile strength of clay filled PA66/ PP composites depends on several factors such as dispersion of 60
PA66 PP
50
PA66/PP
Table 3 Mechanical properties of composites. Sl no.
Sample designation
Tensile strength (MPa)
Strain
9. 10. 11. 12. 13. 14. 15. 16.
PA66 PP PA66/PP 2%NC-PA66/PP 3%NC- PA66/PP 2.5%Gr-PA66/PP 5%Gr-PA66/PP 2%NC + 10%SCFPA66/PP
48.94 18.71 30.65 22 21.7 30.65 30.9 49
0.15 0.21 0.13 0.08 0.06 0.11 0.10 0.07
Hardness (shore-D)
Density (g/cc)
80 41 64 71 74 66 71 75
1.09 0.9034 0.9072 0.9705 1.0282 0.9843 0.9904 1.256
Stress (MPa)
2%NC-PA66/PP
40
3%NC-PA66/PP 2.5%Gr-PA66/PP
30
5%Gr-PA66/PP 2%NC+10%SCFPA66/PP
20 10 0
0
0.05
0.1
0.15
0.2
0.25
Strain Fig. 1. Engineering stress–strain curves generated from tensile tests for the composites.
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the nanoclay in the skin and core layers, interaction of PA66/PP with nanoclay, compatibility of PA66 with PP, interaction of PP with nanoclay and filler–filler interaction of the nanoclay [14]. Increase in tensile strength of PA66/PP blend with addition of nanoclay and short carbon fiber may be due to better interaction between them and uniform distribution of nanoclay and SCF. The effect of different fillers on the elongation at break of PA66/PP composites are also given in Table 3. The elongation at break decreased drastically with the incorporation of different fillers. The brittleness of these composites can be associated with the disappearance of plastic deformation of polymer matrix. In the experimental range, the best mechanical properties were obtained with the composite having 2 wt.% of nanoclay plus 10 wt.% of short carbon fiber with compatibilizer. PA66/PP blend showed lower tensile strength and higher strain. The lower strain is observed in NC + SCF filled PA66/PP composite as compared to other composites. It could be due to addition of nanoclay and short carbon fiber in PA66/PP, reducing the interfacial distance between the matrix and filler. Additionally, the nanoclay particles may provide obstacles to cracks formed in SCF filled PA66/PP, resulting in less convenient crack path and therefore more difficult to fracture. 3.3. Flexural behaviour Flexural properties are of great importance for any structural element. Composite materials used in structures are prone to fail in bending and therefore the development of new composites with improved flexural characteristics is essential. Table 4 lists the flexural strength and modulus of particulate filled PA66/PP composites. It is observed that the flexural strength and modulus increased for NC + SCF filled PA66/PP and decreased for NC and graphite filled systems. It is interesting to note that addition of NC alone lowers the flexural strength drastically. However, in the case of Gr-PA66/PP, the increase in flexural strength is marginal. Among the three fillers selected in the present study, the inclusion of SCF and graphite causes maximum increase in strength of the composite. It could be due to good chemical reaction at the interface between the filler particles and matrix results in better load transfer. 3.4. Impact behaviour While the influence of the filler particles on the tensile strength is rather limited, the influence on the impact strength is significant. The impact strength of different composites recorded during the test are also given in Table 4. It shows that unfilled PA66/PP and graphite filled PA66/PP composite exhibited higher impact strength than that of NC + SCF filled ones. The best performance was achieved with 5 wt.% graphite addition in PA66/PP blend. The addition of fine graphite particles led to 102% higher impact strength than the neat PP. This suggests that for graphite filled
composite subject to impact loading, the interfacial regions are able to resist crack propagation more effectively than the polymer blend. Similar behaviour has also been reported in rubber filled polymer composites. Higher filler loading, usually reduces the impact strength and may be attributed to the agglomeration of filler particles [14]. However, because of lower toughness, the addition of SCF and nanoclay decreased the impact strength of the blend indicating that the compatibility between the polymer matrix and the particulates is poor. The results of impact tests in the present study reveal that graphite can be a promising filler material that would enhance the impact characteristics of polymer composites. Although it is believed that the mechanical properties of a composite are fiber reinforcement dependent, inclusion of fillers in the form of particulates and SCF in PA66/PP influenced the mechanical properties. The results show that improvement in impact strength was achieved with graphite as filler in PA66/PP blend. 3.5. Fractography Very little has been published on the fracture surface morphology of filled polymer nanocomposites by using scanning electron microscopy. It is also known that SEM is not a suitable technique to examine the morphology of polymer nanocomposites. However, in the present work, SEM analysis was used to investigate the effect of the nanoclay, graphite and SCF on the tensile fractured surface morphologies of PA66/PP and their composites. Figs. 2–4 show the SEM pictures of the tensile fractured surface of composite samples. Fig. 2 shows the photomicrograph of the tensile fractured surface of unfilled PA66/PP blend. Big PP particles formed are shown by arrows. They induce some cavitations in PA66 because of which a fibrillar structure appears. The incompatibility of PA66 and PP blends could be observed through PA66 fibril structure and the PP particles pulled out from the PA66 matrix. Similar observations were also found in the literature [24]. Further, the photomicrograph showed brittle fracture of the PA66/PP and pullout of the PP particles. It may be the primary mechanism for fracture. Also, irregular shaped, large PP particles dispersed in the PA66 matrix can be seen. These particles easily detach from the PA66 matrix due to poor interfacial adhesion. The photomicrograph of the tensile fractured surface of 2 wt.% nanoclay filled PA66/PP composite is shown in Fig. 3. The fibrillated morphology observed in PA66/PP blend reduced in this case. The big cavities as shown in Fig. 2 are substantially reduced owing to the presence of MAgPP compatibilizer. However, the presence of
Table 4 Impact strength, flexural stress and flexural modulus values of the composites. Sl no.
Sample designation
Flexural strength (N/mm2)
Flexural modulus (MPa)
Impact strength (J/m)
1. 2. 3. 4. 5. 6.
PA66 PP PA66/PP 2%NC-PA66/PP 3%NC-PA66/PP 2.5%Gr-PA66/ PP 5%Gr-PA66/PP 2%NC + 10%SCF PA66/PP
84.33 30.71 32.06 20.3 19.3 27.4
2654 910 668.80 567.83 567.86 708.28
85 27.82 56 25 18.75 56.25
24.4 51.4
719.25 1009.57
56.25 25
7. 8.
Fig. 2. Scanning electron micrograph of tensile fracture of unfilled PA66/PP blend.
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Table 5 Coefficient of friction of PA66/PP composites under all experimental conditions. Load (N)/composite
Velocity (m/s) 3
PA66/PP 2%NC-PA66/PP SCF + NC-PA66/PP 2.5%Gr-PA66/PP
Fig. 3. Scanning electron micrograph of tensile fracture of 2%NC filled PA66/PP nanocomposite.
4
5
6
20
80
20
80
20
80
20
80
0.54 0.59 0.30 0.33
0.50 0.56 0.25 0.31
0.52 0.58 0.25 0.30
0.50 0.56 0.21 0.27
0.51 0.54 0.24 0.29
0.47 0.52 0.20 0.26
0.50 0.53 0.23 0.25
0.45 0.51 0.20 0.21
6000 m as a function of sliding velocity at different applied loads of 20 and 80 N. This is obtained by dividing the frictional force by the applied normal load and are listed in Table 5. As the applied normal load increases, the coefficient of friction decreases and an increase in sliding velocity results in faster sliding on the counterface, which causes less frictional force and hence a lower coefficient of friction. Differing trends in the values of coefficient of friction were observed for PA66/PP, 2%NC-PA66/PP, 2.5%Gr-PA66/ PP and NC + SCF-PA66/PP composite systems. The variation in wear loss of unfilled and particulate filled PA66/PP composites under different sliding velocities and loads (20 and 80 N) are shown in Figs. 5 and 6 respectively. The experimental results reveal that the wear loss increases linearly with sliding velocity at a constant load. The trend in the wear loss with sliding velocity is the same for all samples irrespective of the load employed. Another interesting feature observed is that the wear loss increases gradually with increase in sliding velocity for
Fig. 4. Scanning electron micrograph of tensile fracture of 2%NC + 10%SCF filled PA66/PP nanocomposite.
PP particles that were detached from the PA66 matrix is obvious from the fracture surface. The reduced fibrillated structure is a direct manifestation of the missing plastic deformation of PA66 matrix due to the incorporation of nanoclay. Fig. 4 shows the photomicrograph of 2 wt.% NC and 10 wt.% SCF filled PA66/PP composite. PP particles are still detached from the PA66 matrix. Further, microcracks can be seen from the fractured surface. Neither a fibrillated morphology nor PP particles detached from the PA66 matrix could be observed. The number of available reinforcing elements is increased due to uniform dispersion of NC and SCF and this in turn enhanced the load carrying capacity. The improvement in mechanical properties is attributed to better interfacial adhesion between the fillers and blend. Further, short carbon fiber may act as a bridge between the phases and thus enhance the load carrying capacity of the blend. This behaviour is reflected from the photomicrograph shown in Fig. 4. The presence of microcracking caused by stress concentration sites (i.e., agglomerated nanoclay and SCF particulates) provides better evidence for the brittleness of the PA66/PP composite.
Fig. 5. Wear loss versus sliding velocity at a load of 20 N and a sliding distance of 6000 m.
3.6. Friction and sliding wear results The friction coefficient of unfilled and particulate filled PA66/PP composites are obtained under the constant sliding distance of
Fig. 6. Wear loss versus sliding velocity at a load of 80 N and a sliding distance of 6000 m.
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2%NC-PA66/PP and NC + SCF-PA66/PP, whereas in the unfilled and 2.5%Gr-PA66/PP, a higher gradient is seen. The wear loss increases with increasing load for all samples and shows an upward trend indicating greater wear loss at high load (Fig. 6). The primary reasons for adding fillers or reinforcing fibers to polymers is to improve their mechanical properties, but the effects on wear rate are not invariably beneficial. Suresha et al. [22–27] investigated the wear behaviour of particulate filled glass/carbon fabric reinforced epoxy composites. The above investigations were concentrated on sliding and three-body/two-body abrasive wear studies. The silica sand was used as dry and loose abrasives in three-body wear tests. In abrasive wear situations, the wear rate depends on the experimental test parameters such as load and abrading distance. The fillers such as cenosphere and graphite were observed to be detrimental to abrasive wear performance. Graphite filler in carbon epoxy composite provides better wear resistance under dry sliding wear tests. In the present study, the highest wear resistance is for NC + SCF filled PA66/PP composites. In previous studies, the effect of particular fillers on mechanical and three-body abrasive wear behaviour of polyamide66/polypropylene nanocomposites has been investigated [28]. Here abrasive wear tests were performed at different loads/abrading distances. It was noticed that particulate fillers especially short carbon fiber improved the mechanical properties and the wear volume of unfilled PA66/PP blend with respect to (a) NC-PA66/PP and (b) NC + SCF-PA66/PP composites respectively show 142% and 216% decrease. The reduction in wear volume of unfilled PA66/PP is related to an inclusion of MAgPP as compatiblizer results in lower interfacial surface tension, finer and stabilized phase morphologies. As shown in Figs. 7 and 8, the wear rate of PA66/PP compos-
Fig. 7. Wear rate of particulate filled PA66/PP nanocomposites at a load of 20 N and a sliding distance of 6000 m.
Fig. 8. Wear rate of particulate filled PA66/PP nanocomposites at a load of 80 N and a sliding distance of 6000 m.
ites with the addition of particulate fillers was clearly lower than that of the blend without particulate fillers. The wear rate of composites increases with increasing sliding velocity and load. The wear of the composites appears to be strongly influenced by the stability of the transfer film on the counterface. The addition of short carbon fibers remarkably enhanced the tribological performance of NC filled PA66/PP. It can be seen that the coefficient of friction was reduced with SCF and the wear rate of the composite was doubly decreased. The addition of short carbon fibers to the nanoclay filled PA66/PP arrests stick–slip and induces large plastic flow at the surface. This appears to stabilize the film, which now interacts with the counterface film in a low adhesion regime resulting in less wear. 3.7. Worn surface morphology The worn surface of the composites was observed by SEM examination to search for a correlation of worn surface and slide wear loss with increasing sliding velocities/loads. Figs. 9–12 show morphology of the worn surface of the PA66/PP blend, 2%NC-PA66/ PP, NC + SCF-PA66/PP and 2.5%Gr-PA66/PP composites at 6000 m, 80 N load at a velocity of 6 m/s test conditions. The photomicrograph of the worn surface of PA66/PP blend is as shown in Fig. 9. Due to mechanical action and the accumulated frictional heat the molecular structure of PA66/PP blend has been partially destroyed. The interfacial bonding strength is weaker for the blend thereby, producing severe plastic deformation. PP has straight molecular chain and relatively lower softening point. The poor adhesion between PA66 and PP results in higher wear loss at higher load and velocity. The photomicrograph of the worn surface of 2.5%NC-PA66/PP showed in Fig. 10. It reveals a network of microcracks and few furrows in the sliding direction due to incorporation of nanoclay. Elastic and plastic deformations on the surface of PA66/PP become the main cause for the wear under the effect of nanoclay inclusion. The repeated deformation causes cracking on the surface or subsurface of the composite. The photomicrograph of the worn surface of NC + SCF-PA66/PP composite is as shown in Fig. 11. The wear rate was reduced by more than twice in magnitude when 10 wt.% of SCF as added. The beneficial effect as a result of SCF reinforcement in mainly due to a reduced ability of ploughing, tearing and other non-adhesive components of wear [29]. At the beginning, the surface is cut by the asperities of the counterpart. Micro-furrows are generated on the surface of the sample, and the transfer film is formed on the surface of the counterpart. The photomicrograph of the worn surface of 2.5% Gr-PA66/PP composite is shown in Fig. 12. It reveals
Fig. 9. Scanning electron micrograph of PA66/PP blend at 80 N, 6000 m and 6 m/s.
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4. Conclusions
Fig. 10. Scanning electron micrograph of 2%NC-PA66/PP at 80 N, 6000 m and 6 m/s.
Fig. 11. Scanning electron micrograph of 10%SCF + 2%NC-PA66/PP blend at 80 N, 6000 m and 6 m/s.
The study shows the successful fabrication of unfilled and filled PA66/PP composites. The structural behaviour of PA66/PP blend and their composites was evaluated using static and quasi- static mechanical tests. The material with 2%NC + 10%SCF-PA66/PP composite showed the most superior mechanical properties with a tensile strength of 50 MPa, hardness 75 (Shore-D), flexural strength of 52 MPa and flexural modulus of 1010 MPa. Further, NC + SCF exhibit strain hardening in addition to increased strength. The superior mechanical properties and the strain hardening exhibited by NC + SCF filled PA66/PP composite can be explained by the increased polymer network density caused by physical entanglements from the dispersion of nanoclay and short carbon fibers in the blend. A significant improvement was found in the toughness by performing impact tests. An enhancement of 102% was measured with fine graphite particles in PA66/PP. In the material with PA66/PP blend, the PP phase was present in the form of large particles in PA66 matrix. The interfacial adhesion between PA66 and PP was very poor. The hardness, density, tensile and flexural properties of the composite are also greatly influenced by the type and content of micro or nano fillers. Hence while fabricating a composite of specific requirements, there is a need for the choice of appropriate filler material and for optimizing its content in the composite. NC + SCF-PA66/PP composite with compatibilizer showed better resistance to dry sliding wear compared to unfilled and filled PA66/PP composites. Combination of short carbon fiber and nanoclay is very effective in reducing the coefficient of friction in sliding against steel counterface. SEM observations throw further light on features such as microcracks, pull out of PP particles, debonding of filler from the matrix and transfer film formation. The wear resistance of the micro/nano filler filled PA66/PP composites can be utilized to systematically design a composite material with a good wear performance. In conclusion, an optimum wear resistance can be expected from a composite consisting of PA66/PP blend and NC + SCF filler with MagPP compatibilizer. These experimental results indicate that in the context of application as a bearing material (journal bearings and antifriction bearings), the performance of PA66/PP blend and NC + SCF filler with MagPP compatibilizer is good and possibly superior to unfilled PA66.PP composites.
Acknowledgments We would like to thank N. Govinda Raju, Chief Executive Officer, Magnum Engineers, Bangalore, for providing the testing facilities. The authors are thankful to the Management and Principal Dr. M.S. Shiva Kumar, The National Institute of Engineering, Mysore, India, for their encouragement. The authors are also thankful to M. Ganesh, Managing Director, GLS Polymers Pvt. Ltd., Bangalore, for extending the fabrication facility for this research work. The authors also acknowledge the contributions of Dr. P.R. Sadananda Rao in preparation of this manuscript.
References Fig. 12. Scanning electron micrograph of 2.5%Gr-PA66/PP blend at 80 N, 6000 m and 6 m/s.
that due to heat generated by friction; destroy the molecular structure of Gr-PA66/PP composite. Fig. 12 also shows a smooth film with microcracks at the centre and the ploughed zone marked by extensive plastic flow and deformation.
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