effect of particle morphology on the properties of polypropylene

Letter

EFFECT OF PARTICLE MORPHOLOGY ON THE PROPERTIES OF POLYPROPYLENE/NANOMETRIC ZINC OXIDE (PP/NANOZNO) COMPOSITES Ong Hui Lin1, Hazizan Md Akil1,* and Shahrom Mahmud2 School of Materials and Mineral Resources Engineering, University Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia. 2 NanoOptoelectronic Research Lab, School of Physics, University Sains Malaysia, 11800 Minden, Penang, Malaysia.

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Author to whom correspondence should be addressed E-mail:[email protected]

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Received 16 April 2009; accepted 9 July 2009

ABSTRACT Polypropylene/nanometric zinc oxide (PP/nanoZnO) composites at 1 wt% nanoZnO content were prepared using melt blending method using a thermo Haake internal mixer. Three different types of zinc oxide (ZnO) with different morphologies were used as fillers. Each composite was subjected to characterization analyses including tensile testing, UV-vis spectroscopy and electron microscopy. The tensile strength, tensile modulus and elongation at break of the PP/nanoZnO composites were observed to be greatly enhanced despite the low filler content (1 wt%). All ZnO-reinforced composites exhibited superior UV absorption characteristic especially for composite specimens reinforced with ZnO morphology rich in nanorods. Keywords: Polymer matrix composites; Nano zinc oxide; Mechanical properties

1. INTRODUCTION Semicrystalline polypropylene (PP), a useful and versatile commodity thermoplastic, could be the most popular matrix for nanocomposites study. To date, PP is widely used in packaging, automotive and aerospace industries. Among the main reasons for the popularity of PP are its structural flexibility, high isotacticity, good mechanical performance, narrow molecular weight distribution and good optical transluence [1-3]. In recent years, the incorporation of nanoparticles in PP matrix has generated intense enthusiasm among polymer scientists due to its promising industrial applications in many facets of technology. Nanocomposites with good filler dispersion offer significant improvements in mechanical, thermal, electrical, optical and physico-chemical properties even at relatively low filler content [4-6]. Zinc oxide (ZnO) is an important optoelectronic material characterize by a wide direct band-gape of 3.37 eV (room temperature) and large excitation binding energy of 60 meV. ZnO is currently applied Advanced Composites Letters, Vol. 18, Iss. 3, 2009

in healthcare, rubber, varistors, paint, ceramics and cosmetics [7-12]. Feng et al. reported that the incorporation of 10 - 12 wt% of nanoZnO into PP matrix can increase the composite surface resistance and electrostatic voltage to very low values of 109 Ω and 250 V, respectively. Significant improvement in wear resistance, tensile strength, impact strength and crystallization were also reported for PP/nanoZnO composites [9, 13-14]. Interestingly, PP/nanoZnO composites possess excellent antibacterial capability against two human pathogenic bacteria such as Staphylococcus aureus and Klebsiella pneumoniae, making it a suitable candidate for food packaging applications [7]. Ammala et al. investigated the degradation behaviour of nanoZnO/PP under UV radiation by comparing the conventional hindered amine light stabilizers (HALS) with ZnO, and they reported excellent resistance to UV degradation for specimens embedded with 2 wt% of nanoZnO [15]. ZnO can be fabricated into various morphologies resembling rods, wires, plates, mallets, drums, belts, mallets, tubes, cages and flowers [11-12]. With re77

Ong Hui Lin, Hazizan Md Akil and Shahrom Mahmud

gards to morphology-induced responses, Mahmud et al. reported that semiconducting varistors made from nanorod-rich ZnO exhibited superior electrical nonlinearity compared to the ones made from grainular-shape ZnO [11]. In this study, 1 wt% of ZnO with different morphological configurations were incorporated into PP matrix using melt mixing process in order to produce polymer composites. Physical, optical, mechanical and morphological test were carried out to evaluate the performance of the composites. 2. MATERIALS AND EXPERIMENTAL 2.1. Materials Isotactic Polypropylene (PP) was purchased from Titan PP polymers (M) with a melt flow index of 1.6 g/10 min (at 230 oC). The density of the polymer is 0.9 g/cm3. Two types of zinc oxide, white seal (ZnO-W) and pharmaceutical grade (ZnO-P, purity: 99.8%) produced by using French-process, were obtained from the industry. Another type of zinc oxide was zinc oxide nanorod (ZnO-N) synthesized from a method known as catalyst-free combust-oxidized mesh (CFCOM) process in that the experimental details can be found in our previous work [10]. The structures of these three types of ZnO are illustrated in Fig. 1. 2.2. Composites preparation and characterization Zinc oxide (ZnO) nanofillers were ground using mortar and pestle followed by sieving (42 µm) to obtain fine and uniform particle size. Prior to the melt processing, ZnO was dried in a vacuum oven for 24 hr at 80 oC. All composites were prepared using melt blending technique in a Thermo Haake Polydrive Internal Mixer fitted with cam blades at a rotor speed of 60 rpm. Mixing time and temperature were held constant at 8 min and 180 oC, respectively. After 4 min of compounding time, ZnO was added into the mixer. The compounded materials were than hot-compressed into plaque of 1 mm thickness at 180 oC for 10 min, followed by cooling for 5 min under pressure. Measurements of the mechanical properties, such as 78

Fig.1: FESEM micrographs of zinc oxide nanostructure (a) ZnO-white seal (ZnO-W), (b) ZnO- pharmaceutical grade (ZnO-P) and (c) ZnO nanorods (ZnO-N).

tensile strength, modulus, yield strength and elongation at break were performed on an Instron 5533 dynamometer, in accordance with ASTM-D638 using a crosshead speed of 50 mm/min and gauge length of 50 mm. Prior to measurements the moulded specimens were cut into a dog bone shape and conditioned at room temperature for 24 hr. All tests were conducted at room temperature (25 oC). Five measurements were conducted for each specimen and Advanced Composites Letters, Vol. 18, Iss. 3, 2009

Effect of Particle Porphology on the Properties of Polypropylene/Nanometric Zinc Oxide (PP/Nanozno) Composites

the results were averaged to obtain a mean value. The morphology of fractured composites surface was examined by using field emission scanning electron microscopy LEO Supra 35 VP model. The specimens were coated with gold-palladium. The UV spectra of PP composites were recorded by Perkin-Elmer Lambda 35 UV-vis spectrophotometer from 190 nm – 1100 nm supported by UV wind lab software. 3. RESULTS AND DISCUSSION 3.1. Mixing torque Fig. 2 illustrates a typical torque versus time data of the PP/nanoZnO composites and compares with the neat PP matrix. As seen in Fig. 2, the initial torque increased rapidly by the incorporation of polymer, which is depicted as a peak at around 40 s. Torque decreased rapidly as soon as temperature of polypropylene increased and melting occurred. After complete melting at around 240 s, nanoZnO was fed to rheomixer. This second peak was proportional to the filler loading. After wetting of the filler by the polymer, the good dispersion of filler in the polymer matrix was obtained. At this point, torque values decreased up to a stable value that is called stabilization torque. Composite reached the stabilization torque at around 400 s. A stable torque is also an indicator of homogenization of filler in the melt [11]. When

stabilization values were compared, it was clearly seen that the incorporation of fibres was accompanied by an increase in the stabilization torque for all the treatments applied. Stabilization torque was 4.2 N m for the neat PP whereas 5.1 Nm for PP/ZnO-N, 6.0 Nm for PP/ZnO-P and 6.3 Nm for PP/ZnO-W, respectively. From the results, PP/ZnO-W composite posed the highest stabilization torque. It can be attributed to the ZnO-W type with quasi hexagonal in-shaped needed more energy in mixing it with PP matrix homogenously. The increase in torque values with incorporating with ZnO nanoparticles can be also explained by enhanced interactions between the fibre and the polymer. This revealed better mechanical response, especially in tensile strength of the composites, which will be explained in section 3.2. 3.2. Tensile properties Tensile properties of PP/nanoZnO composites with various types of nanoZnO fillers (at 1 wt% filler content) are summarized in Fig. 3. In overall, by adding 1 wt% of nanoZnO filler into PP matrix has enhanced the tensile strength, tensile modulus and elongation of the composites. It is clear that PP/ZnO-W composite showed the highest tensile strength (Fig. 3a) and tensile modulus (Fig. 3b) among the other type of ZnO used in this work. Quantitatively, 20.7% increment in tensile strength is recorded for PP/ZnOW composite. Previously, up to 10 wt% of nanoZnO

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Time (second) Fig. 2: Tensile properties of PP and its composites with different types of ZnO filler at 1 wt% filler content. Advanced Composites Letters, Vol. 18, Iss. 3, 2009

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Fig. 3: Tensile properties of PP and its composites with different types of ZnO filler at 1wt% filler content. 80

Advanced Composites Letters, Vol. 18, Iss. 3, 2009

Effect of Particle Porphology on the Properties of Polypropylene/Nanometric Zinc Oxide (PP/Nanozno) Composites

is required to impart 10% increment in tensile modulus [15]. PP/ZnO-P composite also shows a reasonable increase in tensile strength and tensile modulus which is slightly lower than previously reported value. The improvement of tensile strength and tensile modulus may be related to the greater strength of the ZnO filler as compared to PP polymer matrix. The quasi hexagonal and polyhedral structures (rods and drums) of ZnO-W and ZnO-P have allowed the stress effectively transferred to other ZnO nanoparticles or polymer matrix in the polymer matrix, which enhances the strength and stiffness of the composites. The ZnO-W type filler with many types of nanostructures makes it suitable filler with excellent tensile properties without anisotropy [15]. On the order hand, ZnO-N type filler which had smaller average crystal size, tend to agglomerate when mixing with PP polymer matrix resultant lower tensile properties. From engineering point of view, elongation at break is an important parameter describing the rupture behaviour of the composites materials. The addition of filler into polymers used to lower the value of elongation at break, even though the matrix has high impact toughness [17]. Fig. 3c clearly indicates that this is not the case when ZnO filler is used. ZnO-W and ZnO-P type filler were able to increase elongation at break of PP, reaching up to 500%. These fillers have introduced additional crazing and perhaps act as stoppers to crack growth at the same time [18].

UV-visible absorbance spectra of PP polymer and composites of PP are presented in Fig. 4. Composites with different types of ZnO show better absorption in the UV region compared to PP polymer matrix. PP/ZnO-N was the highest absorptance in the UV region among the composites. This is due to the inherent capability of nanoZnO to absorb in the UV region. So, this will impart photo stability to the PP polymer matrix [7]. Simultaneously, the absorptance of PP/ZnO composites in the visible region was much higher compared to that of PP due to the reduced reflectance of ZnO. Rod-like or acicular structures of ZnO are known to be characterized by ±(0001) crystal facets whereby these facets are photo-catalytically active [19]. Photocatalytic levels can be enhanced with acicular structures that possess smaller nanometric diameters and larger aspect ratios [20,21] ; therefore, the exceptionally strong UV absorption demonstrated by PP/ZnO-N sample (Fig. 1c) can be attributed to the relatively higher aspect ratio of ZnO nanorods. Moreover, the higher optical absorption in the visible regime (400-700nm) of PP/ZnO-N sample can also be attributed to structural defects of the nanometric diameter of the ZnO nanorods [19-20]. Fig. 5 displayed the morphology of the tensile fracture surface of PP/ZnO composites. The white spots in Fig. 5b and c represented the ZnO fillers. A coarser appearance with moderate plastic deformation

Fig. 4: Uv-vis absorptance spectra of PP matrix and its composites at 1 wt% filler content. Advanced Composites Letters, Vol. 18, Iss. 3, 2009

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4. CONCLUSIONS ZnO is a promising filler material for composite thermoplastics in that it imparts functional benefits such as superior mechanical properties and enhanced UV absorption. We have demonstrated that by achieving good filler-matrix homogeneity, even a 1 wt% ZnO filler amount is capable of imparting the wonderful properties. Diverse ZnO morphology appears to exhibit different mechanical-optical responses whereby granular ZnO tends to improve mechanical properties whereby acicular (rod-like) morphology tends to contribute superior UV absorption. There is a need to study the underlying characteristics of ZnO morphologies and how they interact with thermoplastics in order to further maximize performance with minimum filler amount.

Fig. 5: Tensile fractograph of PP/ZnO composites at 1 wt% filler content. (a) PP with ZnO-N type filler; (b) PP with ZnO-P type filler and (c) PP with ZnO-W type filler.

can be observed for the fracture surface of PP incorporated with ZnO-W type filler composite. This is again proved that PP/ZnO-W composite shows the best tensile properties among the other type composites. In contrast, the agglomerations of nanoZnO filler occurred in the PP polymer matrix which can be observed in Fig. 5a. The agglomerates will bring down the tensile mechanical properties. 82

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