18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS DEFORMATION MECHANISM OF POLYETHYLENE/CALCIUM CARBONATE NANOCOMPOSITES M. Mohesenzadeh, S. M. Zebarjad*, M. Mazinani Dept. of Materials Science and Engineering, Ferdowsi University of Mashhad, Mashhad, Iran * Corresponding author (Zebarjad@um.ac.ir) Keywords : Deformation Mechanism, PP, CaCO 3 , Nanocomposites, Essential Work Of Fracture 1. Introduction polypropylene using a combination of different Several studies have demonstrated an increase filler particle size and loading. The maximum in toughness of polymer matrix using rigid improvement of impact strength achieved with particles. Bartczak et al . [1] used calcium stearic acid-treated filler having 0.7µm size carbonate (CaCO 3 ) filler particles for which agree closely with the results of Thio et toughening of HDPE. They found that the Izod al. [3] but particle sizes less than 0.7µm tended impact toughness of HDPE could be improved, to aggregate and showed very poor dispersion, depending on the size and loading of CaCO 3 which had a detrimental effect on impact utilized. The jump in HDPE toughness was strength. explained on the basis of Wu's criterion [2]. In the present study, deformation mechanism of Combination of particle size and volume medium density polyethylene/Calcium fraction of the filler allowed the condition of Carbonate (MDPE/CaCO3) nanocomposites in interparticle ligament thickness below a value of the form of films have been investigated . For 0.6µm. Under these conditions transcrystalline this purpose, a number of mechanics and layers around the particles come into contact. microscopy techniques, such as the Essential These layers exhibit low plastic resistance. work of fracture (EWF) technique, transmission When the material percolates through the optical microscopy (TOM) were employed. system, large plastic deformation of interparticle ligaments under impact loading result in a sharp 2 Experimental Procedure brittle-to-tough transition. Thio et al . [3] used CaCO 3 with different particle size. It was found Medium density polyethylene HP3840UA that CaCO 3 particles with an average size of 0.7 with MFI (measured at 1900C and 2.16kg) of µm improved Izod impact energy of 4.2 g/10min and a density of 0.937 g/cm3 used polypropylene. No information was provided as the matrix. Precipitated calcium carbonate regarding the interparticle ligament thickness, nanoparticles were obtained from solvay neither indication was given for the existence or company, France, under a trade name of not of a transcrystallized PP layer around the socal312. The particle sizes were about 70 nm. CaCO 3 particles. The claimed toughening Fig. 1 shows the transmission electron mechanism was plastic deformation of micrograph of the CaCO 3 nanoparticles used in interparticle ligaments, following particle - this study. As it can be seen in this TEM matrix debonding with additional contribution micrograph, particles have an irregular resulting from crack deflection toughening. The morphology. In order to avoid agglomeration smallest particles agglomerated and the largest and better dispersion of CaCO 3 nanoparticles in particles were irregular in shape and size that the matrix, the MDPE and CaCO 3 powder were led to earlier fracture. Zuiderduin et al. [4] mixed in a mixer mill (Retsch MM400) at pre - investigated the toughening of CaCO 3 filled selected mass ratio. The mixing was performed
DEFORMATION MECHANISM OF POLYETHYLENE/CALCIUM CARBONATE NANOCOMPOSITEOSITES at a frequency of 20 Hz for 10 min. Then, the plastic zone indicates that different stages of mixtures were compression molded into films of deformation have taken place during fracture 0.3 mm thickness at 170 0 C and 30 kPa. Deeply experiment [7]. Fig. 5a to 5d show the double edge notched tension (DDENT) transmission optical micrographs (TOM) of the specimens with a total length of H=110 mm, plastic zones formed in DDENT samples. The with a length between the grips of h=60 mm and TOM micrographs clearly indicate that a width of W=40 mm were cut from extensive plastic deformation have occurred in compression molded films. The notches were front of the crack tip. In nanocomposites made using sharp scissors followed by sharpe n ing samples, two distinct regions are visible in front with a fresh razor blade. Tensile test on DDENT of the crack tip, a dark zone and a diffuse zone specimens were performed using a Zwick (Z that extends out around the dark zone and its 250) universal testing machine at room area increases with increasing amount of temperature under a constant crosshead speed of 5 CaCO 3 . The contrast in intensity is due to the mm/min. differences in the deformation mechanisms In order to investigate the deformation micro - within the two zones. The dark zone is called mechanisms, DDENT specimens were tensile intense outer plastic zone (IOPZ) and the diffuse loaded, at a constant crosshead speed of zone is named diffuse outer plastic zone 5mm/min, until plastic zones form in front of (DOPZ). The phenomenon of double plastic the crack tips (Fig. 2). After formation of plastic zone has already been reported by other authors zones, the specimens were unloaded, and the [4-6]. In the case of ethylene-propylene block surface of plastic zones evaluated by a copolymer [6], the spherical micro-voids transmission optical microscope (TOM), under produced in the DOPZ because of the presence cross-polarization conditions. of ethylene phase. However they elongated and coalesced with neighboring voids in the IOPZ that resulted in different extent of stress 3. RESULTS AND DISCUSSION whitening between IOPZ and DOPZ. The photographs of the fractured MDPE The concentration of localized stress near specimen and different nanocomposites are the fracture process zone is higher than that in shown in Fig. 3 As can be seen in these the region away from this region. Beyond the photographs, all the specimens have fractured in fracture process zone, DOPZ was observed a fully ductile manner. Furthermore, the well- where the stress concentration is comparatively defined yielding of the regions neighboring the lower. This is probably the reason for the initial ligaments of the specimens can be formation of double plastic zone in these observed in this series of photographs. The nanocomposite samples. As can be clearly seen development of macroscopic plastic in Fig. 5, the size of DOPZ depends on the deformation zones in the nanocomposite CaCO 3 content in the composite samples. The samples was different from that of the MDPE. A higher CaCO 3 content has resulted in a larger double plastic zone was observed in the DOPZ since the number of stress concentration nanocomposite samples. The double plastic sites increases considerably within the matrix zone is schematically illustrated in Fig. 4. The with increasing filler content. At approximately whole plastic zone in this figure consists of an the end of the linear elastic region, the DOPZ intense outer plastic zone (IOPZ) near the has been formed without having consumed a fracture process zone and a diffuse outer plastic noticeable amount of plastic work. On the other zone (DOPZ) slightly away from it, depending hand, the formation of the IOPZ has taken place upon the intensity of stress whitening in the during the whole fracture experiment during sample. The contrast in intensity observed in the
18 TH INTER 3 RNATIONAL L CONFERE ENCE ON CO OMPOSITE M MATERIAL S w which it h has expand ded into th he previou usly f formed DO OPZ [7]. Th he formati on of dou uble plastic zone p has been pr reviously re eported for the c case of Eth hylene-Prop pylene bloc ck copolym mer [ 7,8], Polyp propylene-E EPDM rubb ber blends [ [9], a and PP/Ca CO 3 /PP-g-M MAH com mposites [1 10]. T The occurre ence of thi s phenome non has be een attributed to a o the elonga ation and c coalescence of microvoids m which inc crease the intensity of stress-whiten s ning in the IOPZ [8]. 4 4. CONCLU USION The EW WF approac ch was succ cessfully us sed Figure 2: DD DENT samp ple used for EWF tests F to o character rize the fr racture beh havior of the medium den m nsity Polyet thylene (M MDPE) as w well as that of th a he compos ite samples s when 1, 2 2.5 and 5wt.% a CaCO 3 nan noparticles are added to M MDPE. It w was shown the CaCO 3 nanopartic cles added into t a the MDPE m matrix in al ll quantities s to d decrease the e specific es ssential wo rk of fractu ure. This detrime T ental effect t of CaCO 3 nanopartic cles w was attribut ted to the i introduction n of a cert tain number of s n stress conce entration si ites within the matrix and m the fact th at the situa ation becom mes much more m e severe w hen greater r amounts of nanaoparticl n les are ad dded to th he compos site giving rise g also to i increase th he chance of particles agg p glomeration n during the e manufactu ure of test specim o mens. Fig gure 3: Fr ractured sp pecimens d during EW WF fra acture expe eriments; M MDPE, and d composite es wi ith 1, 2.5 an nd 5 wt.% C CaCO 3 Fig gure 4: Sc chematic il llustration o of a doubl le Figure 1: F Transmissio on electron n microgra aph pla astic zone (D DPZ) consi isting IOPZ and DOPZ Z s showing the e nano sized d CaCO 3 par rticles 3
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