polypropylene/styrene-butadiene- styrene ...

International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 5, May 2018, pp. 200–216, Article ID: IJMET_09_05_024 Available online at http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=5 ISSN Print: 0976-6340 and ISSN Online: 0976-6359 © IAEME Publication

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POLYPROPYLENE/STYRENE-BUTADIENESTYRENE NANOCOMPOSITES: MECHANICAL PROPERTIES Mubarak Y. A and Abu-Halimeh R Chemical Engineering Department, The University of Jordan, Amman-11942-Jordan Schubert D Polymer Centre, Friedrich-Alexander University Erlangen-Nurnberg, Erlangen-91058-Germany ABSTRACT Polypropylene/styrene-butadiene-styrene (SBS) silica nanocomposites were mixed in a twin screw Brabender Plasticorder, the weight percent of the SBS was varied at (0, 5, 10, 20 and 40), and the silica content was varied at (0, 0.05, 0.1, 0.5, 1 and 2) wt%. The mechanical properties of these nanocomposites were studied using a Tensile Testing and Impact Testing machines. It was observed that increasing the SBS content lead to a drastic improvement in the impact strength, with over a 6-fold increase at maximum. Young's modulus decreased with increasing SBS content, with the highest decrease being approximately 35% at PP/SBS ratios of 60/40, excluding samples having this composition caused the highest decrease to drop to 25%. A proportional correlation was observed between the elongation at break and SBS content; on the other hand, an inversely proportional one was noticed between the yield stress and SBS content. The silica nanoparticles did not significantly enhance any of the mechanical properties. This is most likely due to the poor dispersion of these fumed silica nanoparticles in the Polypropylene/SBS matrix. Therefore, the SBS properties had a greater effect on the enhancement of the impact properties and the alteration of the tensile properties than the silica. Key words: nanocomposites, fumed silica, styrene-butadiene-styrene, polypropylene, impact strength, tensile strength. Cite this Article: Mubarak Y.A, Abu-Halimeh R and Schubert D, Polypropylene/Styrene-Butadiene-Styrene Nanocomposites: Mechanical Properties, International Journal of Mechanical Engineering and Technology 9(5), 2018, pp. 200–216. http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=5

1. INTRODUCTION Polypropylene (PP) is a thermoplastic material with a wide range of applications including automotive, packaging, textile, piping, wire and cable, fiber drawing, film stretching, medical

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Polypropylene/Styrene-Butadiene-Styrene Nanocomposites: Mechanical Properties

devices, blow molding, extrusion, injection molding and other consumer products. It is one of the growing classes of commercial thermoplastics [1-3]. Polypropylene as a semi-crystalline thermoplastic was introduced in the 1950s, since that time it attracts researcher’s interests due to its high strength to weight ratio, it is more rigid than other polyolefin. It has a high melting temperature (about 165 oC), better heat resistance than other low-cost thermoplastics [4-6]. Polypropylene has excellent dielectric properties and high resistance to most alkalines and acids, organic solvents, degreasing agents and electrolytic attack. On the contrary, propylene is less resistant to aromatics, aliphatics and chlorinated solvents [1]. However, it has a very good fatigue resistance and high resistance to cracking; it has low impact properties especially at low temperatures [7]. The main problem with polypropylene is that it is deficient in its impact properties especially at temperatures below 0°C [8]. The impact resistance of a polymer is critical for certain applications; such as plastic storage containers, toys, plastic bottles, packaging, piping, components for automobiles and casing for personal computers. It is one of the most important mechanical properties, there are many ways a polymer's impact resistance may be enhanced; such as the addition of a second polymer with good impact properties to form a copolymer, the addition of a plasticizing agent, altering the crystallinity of the polymer and the addition of a rubber phase to the main polymer [9]. Many types of research and experiments have been carried out in order to enhance polypropylene’s mechanical properties. Gupta and Purwar [10] reported studies on the tensile and impact properties of several binary and ternary blends of polypropylene (PP), styrene-bethylene-co-butylene-b-styrene triblock copolymer (SEBS), high-density polyethylene (HDPE), and polystyrene (PS). The ternary blend PP/SEBS/HDPE showed properties distinctly superior to those of PP/SEBS/PS or the binary blends PP/SEBS and PP/HDPE. D'Orazio, et al. [11] studied the effect of ethylene-co-propylene (EPR) on the impact properties of polypropylene. They found that the range of EPR particle size effective for PP toughening was dependent on test temperature; the effective particle size range was narrower at temperatures lying between the glass transition temperatures of EPR and PP than that at room temperature. Sanadi et al. [12] used recycled newspaper fibers as reinforcing fibers to improve the impact and tensile properties of polypropylene. A high energy thermokinetic mixer was used to blend the fibers with polypropylene; the resulting blends were then formed into impact testing samples and tensile testing samples via injection molding. Due to the fact that PP is hydrophobic whereas the fibers are highly polar, maleic anhydride-grafted polypropylene (MAPP) and acrylic acid-grafted polypropylene (AAPP) were used to improve the interaction between the initial materials. The addition of MAPP showed to give better results than when adding AAPP. Rong et al. [13] used an irradiation grafting method so as to modify nano-inorganic particles; these particles were then added to polypropylene in order to improve its mechanical performance. They found that the reinforcing and toughening effects of the nanoparticles on the polymer matrix could be fully brought into play at a rather low filler loading in comparison to conventional particulate filled composites. This technique of modifying nanoparticles has many advantages, it is simple, low in cost, easy to be controlled and has broader applicability than other techniques. Hattotuwa et al. [14] compares the mechanical properties of rice husk powder filled polypropylene with talc filled polypropylene composites. Unmodified and ground talc and http://www.iaeme.com/IJMET/index.asp

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Mubarak Y.A, Abu-Halimeh R and Schubert D

rice husk (RHP) fillers were compounded with polypropylene (PP) separately in a Brabender plasticorder internal mixer at 180 °C and 50 rpm in order to obtain composites, which contain 0–60% php (per 100 part of polymer) of filler at 15% intervals. The mechanical properties of the composites with reference to filler type and filler loading were investigated. It was noticed that 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. The RHP composites exhibited lower yield strength, Young’s modulus, flexural modulus, and higher elongation at break than talc composites. Scanning electron microscopy (SEM) was used to examine the structure of the fracture surface and to justify the variation of the measured mechanical properties. Rana et al. [15] studied the properties of short jute fiber reinforced composites. The jute– PP granules were made in a K-mixer (a high shear mixer granulator) and molded using an injection molding machine to produce ASTM test pieces. In general, it was found that the increase in toughness was always accompanied with a decrease in tensile properties, both impact and tensile properties show an increasing trend with the compatibilizer and it was also found that increasing the fiber loading caused an increase in impact strength. Garcia et al. [16] studied the properties of polypropylene/silica nanocomposites. The nanocomposites were synthesized using twin-screw extruders and their properties were studied using two different inorganic fillers: colloidal (sol) and powder silica nanoparticles. Addition of colloidal silica to the polymer matrix produced good filler dispersion while the use of powder silica resulted in aggregated silica particles in the polymer matrix. There was no noticeable improvement in the mechanical properties when powder silica was added to the pure polymer. On the contrary, the presence of silica-sol nanoparticles in the polymer matrix led to an increase of both Young modulus and impact strength. Hernández et al. [8] studied the impact properties of polypropylene /styrene-butadienestyrene block copolymer (PP/SBS) blends. Concentrations of SBS were 15, 30 and 40 %wt. Special reference was made to the influence of the blend ratio and the vulcanization method (dynamic and static). Impact measurements exhibited that pure PP has extremely low impact strength. It was concluded that improved impact strength can be achieved by blending PP with SBS via dynamic vulcanization. However, static vulcanization proved to be not as efficient as dynamic vulcanization for improving impact resistance. Yang et al. [17] investigated the influence of impact modifier on the microstructure and physico-chemical and mechanical properties of polypropylene crystallized at elevated pressures. The impact modified polypropylene referred as thermoplastic olefin (TPO) was characterized by a small spherulite size in the range of 1–8 μm and the effect was retained when nanoclay was introduced into TPO. The impact strength of TPO and TPO–nanoclay system are similar and significantly higher than the polypropylene system, a behavior attributed to the refinement of spherulite size. The study demonstrated that the nanoclay in TPO retained the impact strength of TPO and controlled the dispersion of elastomer. Iwamoto et al. [18] prepared polypropylene/ethylene-butene composites by mixing it with wood flour and lignocellulose nanofibers. The authors reported that the addition of ethylenebutene increased the impact strength of polypropylene but decreased the stiffness. Also, wood flour and lignocellulose nanofiber-reinforced composites showed significantly higher flexural moduli and slightly higher flexural yield stresses than did the ethylenebutene/polypropylene blends. It is also reported that the wood flour composites exhibited brittle fractures during tensile tests and had lower impact strengths than those of the ethylene-


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butene/polypropylene blends while the addition of the lignocellulose nanofibers did not decrease the impact strength of the ethylene-butene/polypropylene blends. SBS is a thermoplastic elastomer and due to its low glass transition temperature (T g =80°C), when added to polypropylene it causes the glass transition temperature of the blend to be less than that of pure polypropylene (Tg = 0°C), this ensures greater toughness and better impact properties [19]. Thermoplastic rubbers have the ability to elongate when stretched and return to their original shape upon relieving the applied stress, this property is also transferred to polypropylene upon mixing. The addition of inorganic fumed silica nanoparticles to the PP/SBS composites will also be studied; these nanoparticles should increase the toughness and scratch resistance of the polypropylene through altering the crystallinity of the original polypropylene. The objective of this research is to study the mechanical properties of the polypropylene/styrene-butadiene-styrene composites with and without the addition of fumed silica nanoparticles. The thermal properties of interest are crystallization and melting temperatures in addition to the percentage of crystallinity were published in another paper [20]. What distinguishes the present work is the fact that two approaches to improve the impact and other mechanical properties of polypropylene will be combined; the addition of an elastomer and the altering of crystallinity, by generating a higher number of nucleation sites. To achieve this, polypropylene/styrene-butadiene-styrene fumed silica nanoparticles composites are prepared and analyzed.

2. MATERIALS & EXPERIMENTAL PROCEDURES 2.1. Materials The polypropylene used was Molpen HP525J Homopolymer grade, having a melting temperature between (160-163) oC and a melt flow index of 3 g/10 min [21]. SBS block polymer (D1102) a synthetic replacement for rubber with a styrene/rubber ratio of 28/72, by Kraton Polymers was used in this study. It is supplied in a solid, white, and odorless form, with a typical density between 880-895 kg/m3. It has a glass transition temperature of -80 oC and processing temperature between (150-200) oC and a melt flow index of 11 g/10 min [22]. Fumed silica nanoparticles were supplied by Chempur–Germany and used as is. Fumed silica consists of microscopic droplets (80) nm of amorphous non-porous silica fused into branched, chainlike, three-dimensional secondary particles which then agglomerate into tertiary particles. It has a specific surface area of 440 m²/g and a molecular weight of 60.09 g/mol [23, 24].

2.2. Preparation of Composites A Haake Poly Drive Twin Screw Brabender Plasticorder was used to prepare thirty-one blends having various compositions of PP/SBS and fumed silica nanoparticles as shown in Table 1. The materials were added in sequence through a hopper and the mixing was carried out for 5 minutes, at a temperature of 220 °C and a speed of 60 rpm. The resulting blend was taken from the cavity of the Brabender, left to naturally cool, then its size was reduced using a Wanner 3-Blade Cutter [25]; so as to carry-out the required analyses such as DSC and PLM where small particles are needed [26], and facilitate preparation of mechanical testing samples. Two identical batches of each composition were prepared, having an average weight of 60 g per batch.


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Mubarak Y.A, Abu-Halimeh R and Schubert D Table 1 Compositions of PP/SBS fumed silica nanocomposites Sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

PP [%] 100 0 95 90 80 60 100 95 90 80 60 100 95 90 80 60

SBS [%] 0 100 5 10 20 40 0 5 10 20 40 0 5 10 20 40

Silica [%] 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.05 0.05 0.05 0.05 0.10 0.10 0.10 0.10 0.10

Sample

PP [%]

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

60 100 95 90 80 60 100 95 90 80 60 100 95 90 80 60

SBS [%] 40 0 5 10 20 40 0 5 10 20 40 0 5 10 20 40

Silica [%] 0.10 0.50 0.50 0.50 0.50 0.50 1.00 1.00 1.00 1.00 1.00 2.00 2.00 2.00 2.00 2.00

2.3. Preparation of Tensile Samples Dumbbell-shaped tensile samples were prepared using a Thermo Scientific, Haake Mini Jet II Injection machine [27]. The composite granules were inserted into the Mini Lab cylinder, melted at a temperature of 220°C then this cylinder was transferred to its position in the Mini Jet II machine; this machine was operated according to the following conditions: Mould Temperature [60 °C], Operating Temperature [240 °C], Injection Pressure [500 bar], Injection Time [5 sec], Holding-up Pressure [200 bar] and Holding-up Time [5 sec]. The prepared tensile samples were analyzed via a Hounsfield, H50K-S UTM Tensile Testing machine, using a transducer load force of 5 kN and a testing speed of 10 mm/min. The dimensions of each sample were measured via a digital caliper, each sample was clamped, tested, and the resulting curve was taken for analysis. Approximately 10 replicates of each sample were tested; the average of these results was used to represent the final result.

2.4. Preparation of Impact Samples Impact testing samples were prepared by hot compression molding (Carver Auto Series Press), using a (65×65×3) mm steel square-shaped mold and a heat-resistant over-head projector transparency sheets to form a non-stick layer between the composites and the compression platens. The samples were compressed at 220°C and 158 bars for 4 minutes then naturally cooled; they were then cut using a VLS 6.6 Versa Laser System into (63.5×12.7×3) mm samples [28]. A manual Notcher (Ceast 6530) was used to create a 45, 2.5 mm notch at the center of each sample [29]. The prepared impact samples were analyzed via a 6545 Ceast Impact Tester. The sample was centered for testing, and a 7.5 J pendulum hammer was used to hit the sample. The energy needed for breaking each of the samples was recorded for further calculations. Approximately 8 replicates of each sample were tested; the average of these results was used to represent the final result.


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3. TENSILE AND IMPACT PROPERTIES OF POLYPROPYLENE 3.1. Effect of Fumed Silica Figures 1 shows the stress-strain curves for both virgin polypropylene and PP/fumed silica nanocomposites with no SBS addition.

Tensile Stress [MPa]

35 Silica: 0%

30

Silica: 0.05%

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Silica: 0.1%

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Silica: 0.5% Silica: 1%

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Silica: 2%

10 5 0 0

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200 Strain [%]

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35 34.5 34 33.5 33 32.5 32 31.5 31 30.5

Yield stress

0

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0.58 0.57 0.56 0.55 0.54 0.53 0.52 0.51 0.5 0.49

Young's modulus

1 1.5 Fumed silica weight %

2

Young's Modulus [GPa]

Yield tensile stress [MPa]

Figure 1 Stress-strain curves of PP/fumed silica nanocomposites with 0 wt% SBS.

2.5

Figure 2 Yield stress and Young’s Modulus of polypropylene nanocomposites as a function of fumed silica wt %.

Figure 2 shows that the increase of silica nanoparticles caused polypropylene's yield stress to decrease with no specific trend while Young’s modulus value decreases then increase but still lower than the value of virgin polypropylene. On the other hand, Fig. 3 reveals that the addition of fumed silica nanoparticles increased the elongation at break by about 1.5 folds even at a very low weight % of fumed silica.


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Figure 3 Elongation at break of polypropylene nanocomposites as a function of fumed silica wt%.

The results of the impact testing are shown Fig. 4. The addition of fumed silica nanoparticles did not change polypropylene’s impact strength that much. The obtained values ranged between 2.29 and 3.17 kJ/m2, and in comparison with the impact strength of the virgin PP these values are 18% less and 12% greater, respectively.

Impact strength [kJ/m2]

12 10 8 6 4 2 0 0

0.5


2

Figure 4 Notched Izod Impact Strength of polypropylene nanocomposites as a function of fumed silica weight %

3.2. Effect of Styrene-Butadiene-Styrene Figure 5 shows the stress-strain curve for the virgin polypropylene and the stress-strain curves of each group of samples having the same PP/SBS content with the absence of the fumed silica nanoparticles. The figure reveals the strong effect of the high weight % of SBS (40 wt%) on the yield and the ultimate strength of PP composite. Figures 6 and 7 show that the increase of SBS content had a similar effect to the fumed silica nanoparticles on the polypropylene but with a very clear trend. The ability of the


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thermoplastic rubbers (SBS) to elongate when stretched is also transferred to polypropylene when mixed as can be concluded from Fig. 6.


35 30

PP/SBS: 100/0

25

PP/SBS: 95/5 PP/SBS: 90/10

20

PP/SBS: 80/20

15

PP/SBS: 60/40

10 5 0 0

100

200

300

400

Strain [%] Figure 5 Stress-strain curves of PP/SBS nanocomposites with 0 wt% fumed silica nanoparticles.

0.6 Yield stress

Young's modulus

0.55

35

0.5

30

0.45 25

0.4

20

0.35

15

Young's Modulus [GPa]


40

0.3 0

5

10

15

20 25 30 SBS weight %

35

40

45

Figure 6 Yield stress and Young’s Modulus of polypropylene composites as a function of SBS wt%.

The strong enhancement of polypropylene’s impact strength by the addition of SBS is presented in Figure 8. It is clearly noticed that an addition of 40 wt% of SBS to polypropylene increased its impact strength from 2.82 to 12 kJ/m2 (i.e. 320 %). Since SBS poses a much higher impact strength compared with PP, then increasing the weight % of SBS within PP composites led this high increment.


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Elongation at break [mm]

900 800 700 600 500 400 300 0

5

10

15

20

25

30

35

40

45

SBS weight % Figure 7 Elongation at break of polypropylene composites as a function of SBS wt%.


12 10

8 6 4 2 0 0

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35

40

45

SBS weight % Figure 8 Notched Izod Impact Strength of polypropylene composites as a function of SBS weight %.

3.3. The Combined Effect of SBS and Fumed Silica Nanoparticles Figures 9 and 10 show PP composites stress-strain curves when combined effects of SBS and fumed silica nanoparticles are joined. As can be predicted from Fig. 9, the presence of small amount of the fumed silica nanoparticles within the PP/SBS composite unify the behavior of the composite under stress and almost all stress-strain curves were identical. In comparison with Figure 5, it seems the addition of 1 wt% of fumed silica nanoparticles to the PP/SBS composite reduce the effect of SBS as shown in Fig. 10.


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25 Silica: 0% Silica: 0.05% Silica: 0.1% Silica: 0.5% Silica: 1% Silica: 2%

20 15 10 5 0 0

100

200 Strain [%]

300

400

Figure 9 Stress-strain curves of 60/40 PP/SBS nanocomposites as a function of fumed silica wt %.


35 30

PP/SBS: 100/0

25

PP/SBS: 95/5

20

PP/SBS: 90/10

15

PP/SBS: 80/20 PP/SBS: 60/40

10 5 0 0

100

200 Strain [%]

300

400

Yield strength [MPa]

Figure 10 Stress-strain curves of PP/1.0 wt% fumed silica nanocomposites as a function of SBS wt %.

36 34 32 30 28 26 24 22 20 18

SBS wt%

0 5 10 20 40

0

0.5


2

Figure 11 PP/SBS nanocomposites, yield tensile stress as a function of fumed silica weight %.


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Figures 11 to 14 represent the variation of the PP/SBS composite’s yield stress, Young’s modulus, elongation at break, and impact strength as a function of fumed silica weight %. 0.6 Young's mdulus [GPa]

SBS wt%

0.55

0 5 10 20 40

0.5 0.45 0.4 0.35 0

0.5


2

Figure 12 PP/SBS nanocomposites, Young’s modulus as a function of fumed silica weight %.


1000

SBS wt%

900

0 5 10 20 40

800 700 600 500 400 0

0.5 1 1.5 Fumed silica weight %

2

Figure 13 PP/SBS nanocomposites, elongation at break as a function of fumed silica weight %.

The succeeding points show the behavior of the composite tensile and impact properties at varying fumed silica content: 

As the silica content increases Young's modulus increases slightly, but remains higher at zero silica content.



The yield tensile stress and strain seem to be nearly constant in each group, meaning that the silica content has no effect.



Although the elongation at break for PP increases by increasing the amount of fumed silica nanoparticles added another behavior seems to exist if the SBS is present within the composite. As shown if Fig. 13, the elongation at break for PP/SBS composites increases initially by the addition of low quantities of fumed silica nanoparticles then decreases at high quantities.


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Polypropylene/Styrene-Butadiene-Styrene Nanocomposites: Mechanical Properties Figure 14 shows that the silica content increased the impact strength slightly but with no defined trend. The maximum impact strength was noticed at a composition of 60/40/2 PP/SBS/silica, with a 526 % (over 6 times) higher impact strength than pure polypropylene.




18 16 14 12 10 8 6 4 2 0

SBS wt%

0 5 10 20 40

0

0.5


2

Figure 14 Notched Izod Impact Strength of PP/SBS nanocomposites as a function of fumed silica weight %

The mechanical properties of PP/fumed silica composites as a function of SBS weight % are shown in Fig. 15 to 18. From Fig. 15 to 18 it is seen that at constant silica content: 

The yield stress decreases whilst the yield strain increases with increasing SBS content.



Young's modulus decreases with increasing SBS content.



The addition of SBS increased the elongation at break - but with no specific trend.


36 34

Silica wt%

32

0

30

0.05

28

0.1

26

0.5

24

1

22

2

20

18 0

5

10


30

35

40

Figure 15 PP/fumed silica nanocomposites, yield stress as a function of SBS weight %.

It can be seen, from Fig. 18 that the impact resistance of polypropylene increased upon addition of SBS, and the absorbed energy increased linearly with increasing rubber content.


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This is due to the toughening effect of elastomers as "the presence of soft and flexible particles allows the absorption of more energy during fracture, so the material impact strength rises" [8].

Young's modulus [GPa]

0.6 Silica wt%

0.55 0.5

0 0.05

0.45

0.1 0.5

0.4

1

0.35

2

0.3 0

10

20 SBS weight %

30

40

Figure 16 PP/fumed silica nanocomposites, Young’s modulus as a function of SBS weight %.

It can be concluded that the silica content does not seem to significantly affect any of the tensile properties; proving that the SBS content has more influence on yield stress, yield strain, Young's modulus, elongation at break, and impact strength.


1000 900

Silica wt%

800

0

700

0.05

600

0.1

500

0.5

400

1

300

2

200 0

5

10


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35

40

Figure 17 PP/fumed silica nanocomposites, elongation at break as a function of SBS weight %.


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18 16 14 12 10 8 6 4 2 0

Silica wt%

0 0.05 0.1 0.5 1 2

0

5

10

15

20

25

30

35

40

45

SBS weight % Figure 18 Notched Izod Impact Strength of PP/fumed silica nanocomposites as a function of SBS weight %.

4. DISCUSSION OF RESULTS 4.1. Effect of Silica Nanoparticles on the Mechanical Properties of Polypropylene It is obvious, opposite to expectation; the silica nanoparticles did not significantly enhance any of the properties. Although the polarized light photomicrographs did not show any agglomeration or precipitation of the fumed silica nanoparticles within PP/SBS matrix [20], it seems that the dispersion of the nanoparticles is still poor and could not overcome the deterioration in the mechanical properties caused by the addition of SBS. The poor dispersion of fumed silica nanoparticles may attributed to the hydrophobic nature of PP, which gives rise to a significant problem in enhancing the adhesion between te hydrophilic fumed silica nanoparticles and the matrix, creating poor bond strength between both phases. The use of acidified aqueous nanoparticles or coupling agents could be the optimum way to overcome this problem [16].

4.2. Effect of SBS on the Mechanical Properties of Polypropylene This study illustrates that increasing the SBS content improves the impact strength drastically, with over a 6-fold increase at maximum. This makes sense as the elasticity of the SBS leads to the ability to absorb more energy before fracture. The reduction in tensile properties can be mostly attributed to a decrease of the overall crystallinity content with increasing SBS content [26], whereas the increase in both yield strain and elongation at break is due to the extremely high elasticity of the SBS. Hernández, et al. [8] studied the effect of SBS on the mechanical properties of polypropylene using two different blends; dynamically vulcanized (DV) and statically vulcanized (SV). (DV) blends are those that are prepared by the simultaneous curing of the SBS and its fine dispersion in a molten polypropylene via intensive mixing and kneading; similar to the preparation of composites for this study. Whereas with (SV) blends the SBS is previously cured by means of pressure and temperature and then grounded and mixed with polypropylene. It is reported in the study that increasing the rubber content from 15% to 30% in DV blends, originated a rise in impact energy of 47%, while the same increase in SV blends did not produce changes in impact energy. Similar results were obtained by Saroop, et http://www.iaeme.com/IJMET/index.asp

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al. [30] for PP/SBS dynamically cured blends. The authors found that the impact strength increased gradually with increasing SBS content. Young's modulus decreased with increasing SBS content, as expected and as stated by Hernández, et al. [8], this is most likely due to the rule of mixture behavior (meaning that a mixture prepared of two or more components will possess properties of each component, furthermore a higher concentration of a specific component results in a mixture having attributes closer to that component) [31]. The highest decrease of around 35% was seen at PP/SBS ratios of 60/40, excluding samples having this composition the highest decrease becomes close to 25%. The increase in SBS content caused the elongation at break to increase and the yield stress to decrease (with a maximum drop of 44%), this conduct may also be attributed to the rule-of mixture behavior. In closing, these nanocomposites are seen to be suitable for the manufacture of appliances requiring low weights and elevated impact resistance features. The optimum composition of PP/SBS/silica nanoparticles is to be chosen taking into account the required tensile properties; such as Young's modulus, the yield stress and elongation behavior.

5. CONCLUSIONS 

SBS is an elastomer, its addition to polypropylene logically resulted in the increase of its impact resistance. The silica content increased the impact strength slightly but with no defined trend.



As the concentration of SBS increased, the yield strain and elongation at break both increased, whilst Young's modulus and the yield stress decreased in value.



As the concentration of fumed silica increased, Young's modulus increased slightly but remained higher at zero silica content. The yield stress decreased while the strain and elongation at break increased slightly and followed no trend.



From the previous three points, it can be concluded that the SBS properties had a greater effect on the enhancement of the impact properties and the alteration of the tensile properties than the silica content and the morphology of the resulting spherulites. The reduction in the tensile properties can be mostly attributed to a decrease of the overall crystallinity content with increasing SBS content, whereas the increase in both yield strain and elongation at break is due to the extremely high elasticity of the SBS.

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