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RETRACTED: Preparation and characterization of poly(vinyl chloride) calcium phosphate nanocomposites
Materials Science and Engineering B 168 (2010) 231–236
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
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Preparation and characterization of poly(vinyl chloride) calcium phosphate nanocomposites Chetan B. Patil a,∗ , Priyanka S. Shisode a , Uday R. Kapadi b , Dilip G. Hundiwale b , Pramod P. Mahulikar b a b
S.S.V.P.S.L.K. Dr. P. R. Ghogrey Sciences College, Dhule, 424 001, M.S., India School of Chemical Sciences, North Maharashtra University, Jalgaon 425 001, M.S., India
a r t i c l e
i n f o
Article history: Received 2 August 2009 Received in revised form 24 November 2009 Accepted 6 December 2009
1. Introduction
Calcium phosphate [Ca3 (PO4 )2 ] was synthesized by in situ deposition technique and its nano-size (47–97 nm) was confirmed by Transmission Electron Microscopy (TEM). Composites of the filler Ca3 (PO4 )2 (micro and nano) and the matrix poly(vinyl chloride) (PVC) were prepared with different filler loadings (0–5 wt.%) by melt intercalation. The Brabender torque rheometer equipped with an internal mixer was used for the preparation of composites of different formulations. The effect of nano- and microCa3 (PO4 )2 content on the structure and properties of composites was studied. The nanostructures were studied by wide angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM). The mechanical, thermal and dynamic mechanical properties of PVC/micro- and nano-Ca3 (PO4 )2 composites were characterized using Universal Testing Machine (UTM), Thermo Gravimetric Analyzer (TGA) and dynamic mechanical analyzer (DMA). The thermal analysis results showed that the first thermal degradation onset (Tonset ) of PVC/nano-Ca3 (PO4 )2 composites was lower as compared with corresponding microcomposites and higher than that of pristine PVC. However, the tensile strength was found to be maximum at 1% of nano-Ca3 (PO4 )2 and again decreased with increasing loading of nano-Ca3 (PO4 )2 . Further storage modulus of PVC/micro- and nano-Ca3 (PO4 )2 composites was decreased with increasing loading while the glass transition temperature increased marginally. © 2009 Elsevier B.V. All rights reserved.
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Keywords: Poly(vinyl chloride) Calcium phosphate Nanoparticles Transmission Electron Microscopy (TEM) Wide angle X-ray diffraction (WAXD) Tensile strength Thermal stability (TGA), Dynamic mechanical properties (DMA)
a b s t r a c t
Polymer composites represent an important class of engineering materials. The incorporation of inorganic fillers into thermoplastics has been widely practiced in industry to extend them and to improve certain properties. One primary purpose of adding inorganic fillers to polymers is cost reduction. The use of fillers has been also a common practice to enhance the mechanical properties of thermoplastics, such as heat distortion temperature, hardness, toughness, dimension stability, stiffness, and mold shrinkage. The effects of filler on the mechanical and other properties of the composites depend strongly on filler origin, particle shape and size, aggregate size, the fraction of filler, surface characteristics and degree of dispersion. Most of the studies revealed that addition of rigid, inorganic fillers to polymers generally resulted in a significant decrease of toughness compared to neat polymers. There are, however, several studies demonstrating an increase in toughness with rigid particulate fillers in certain composite systems such as, for example, filled polypropylene (PP) and filled polyethylene
∗ Corresponding author. Tel.: +91 257 225 7434/9423777333; fax: +91 257 225 8403. E-mail address: [email protected] (C.B. Patil). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.12.034
(PE). An impressive increase of impact strength was reported by the literature for PE filled with calcium carbonate (CaCO3 ) particles [1–4]. In the recent decade, organic–inorganic nanocomposites have attracted a great interest to scientists, because nanocomposites have emerged as a very efficient strategy to upgrade properties of synthetic polymers to the point where performances exceed largely the ones of conventional composites. Nanocomposites could dramatically induce an improvement in mechanical and electrical properties, heat resistance, radiation resistance, and other properties as a result of the nanometric scale dispersion of the filler in the matrix. Different nanometric compounds such as CaCO3 , SiO2 , TiO2 and ZnO have been used to prepare the nanocomposites with various polymers, however, relatively little attention has been paid to poly(vinyl chloride) (PVC) materials [5–12]. PVC is an important commercial thermoplastic, has been studied and used widely in the industrial fields such as electrical insulators, plastics moldings, and building materials for many years. However, because of some inherent disadvantages, such as low thermal stability and brittleness, PVC and its composites are subjected to some limitations in certain applications. Recently, the development of polymer nanocomposites may present a new way to improve the performance of PVC [13–16]. When conventional toughening modifiers such as CPE, ACR, and NBR are replaced with nanoparticles
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2. Experimental 2.1. Materials
Calcium chloride, ammonium phosphate were of analytical grade and poly(ethylene glycol) (PEG; MW 6000 g) was procured from s. d. Fine Chem. Ltd., Mumbai, India and used for the synthesis of nanoparticles of calcium phosphate. The poly(vinyl chloride) (PVC) of grade (57GERO68) was obtained from Reliance Industries Ltd., Mumbai, India, as a matrix. Processing aid (PA 20), lubricant processing aid (LI 20), Glyceril monostearate as internal lubricant (LUB 11), PE wax as external lubricant and organotin stabilizer were obtained from Supreme Industries Ltd. Jalgaon, India. Calcium phosphate (5 m) is of commercial grade procured from s. d. Fine Chemicals, Company, Mumbai, India and was used as filler without any treatment. 2.2. Preparation of nanoparticles
ites were obtained in the form of lumps. These lumps of composites then crushed to the coarser particles (approximately 3–4 mm size) that were suitable for injection moulding to get tensile and impact testing specimens. 2.4. Characterization The particle size and morphology of the synthesized nanocalcium phosphate particles were studied using Transmission Electron Microscope (TEM, Philips Tecnai-20) at an accelerating voltage of 200 kV. Sample of calcium phosphate was dispersed in acetone with ultrasonic wave for sufficient 12 h before analysis for particle size. The structure of PVC–Ca3 (PO4 )2 nanocomposites was characterized by wide angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM). WAXD was performed on X’pert-Pro PANAlytical diffractometer (Philips) using Cu K␣ radiation at generated voltage of 40 kV and current of 30 mA at 25 ◦ C. The diffractions were scanned from 0◦ to 40◦ in the 2 range in 0.01◦ steps with a continuous scan. To study the dispersion of filler and morphology of composites, a tensile fractured surface was coated with platinum and observed on JEOL 6360 scanning electron microscope (SEM) at an acceleration voltage of 10 kV to obtain the scanning images. A thermogravimetric analyzer (TGA; Shimadzu, Japan, model: TGA-50/50H) was used to analyze thermal characteristics of the PVC/micro- and nano-Ca3 (PO4 )2 composites. The composites were heated from room temperature to 550 ◦ C at the rate of 5 ◦ C/min under nitrogen stream. The onset temperature and the weight loss during thermal degradation of composites were recorded and analyzed. Tensile strength was determined by subjecting dumb-bell shaped specimens (in accordance with ASTM D-638) to a Universal Testing Machine (UTM 2302, R & D Equipment, Mumbai, India). The specimens were conditioned for 24 h prior to testing. The load cell of 3000 kg and a crosshead speed of 50 mm/min were employed. The dynamic mechanical analysis (DMA) was performed using a dynamic mechanical analyzer (Gabo Eplexor, Swiss) at fixed frequency rate of 10 Hz in a temperature range from 30 to 120 ◦ C with a heating rate of 2 ◦ C/min.
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such as CaCO3 or Ca3 (PO4 )2 , the fracture behavior is expected to be changed [17–26]. In the present study, the nanoparticles of Ca3 (PO4 )2 , were synthesized by in situ deposition technique and their particles size was confirmed with TEM. The melt intercalation of PVC with nano- and commercial Ca3 (PO4 )2 were carried out using Brabender Plastograph EC, which gave data on rheological behavior. The composites so formed were tested for their physical, mechanical, thermal and structural properties and morphology of tensile fractured surfaces was also investigated [27–35].
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The nano-sized calcium phosphate filler particles were synthesized using in situ deposition technique which is actually matrix mediated growth and control technique or it is also called as in situ deposition technique. The calcium phosphate is prepared in situ of polymer (PEG) which acts as a growth medium as described below [9–11]. Calcium chloride (111 g, 1 m) was dissolved in distilled water (140 mL) and PEG (372 g, 0.062 m) was separately dissolved in hot distilled water (300 mL). The solutions were mixed properly and gently digested for 12 h. Solution of ammonium phosphate (115 g, 1 m) in distilled water (250 mL) was then added slowly with stirring to the complex and the nanoparticles formed were allowed to digest overnight. The nanoparticles formed were filtered (Whatman filter paper no. 41), washed thoroughly with distilled water, till freed from PEG traces. The prepared filler was then dried at 110 ◦ C for 2 h. Before compounding, the filler was heated at about 250 ◦ C for removing traces of moisture. 2.3. Preparation of composites
The PVC compounds were formulated with 2.5 phr of processing aid (PA 20), 2.5 phr of lubricating processing aid (LI 20), 3.5 phr of glyceril monostearate internal lubricant (LUB 11), 3.5 phr of organotin stabilizer, 2.0 phr of PE wax as external lubricant and fillers both nano- and micro-calcium phosphate with variable concentration (0–5%). These formulations were dry blended in domestic mixer for 15 min. For preparation of composites the pre-powder of these formulations was melt intercalated using Brabender Plastograph EC equipped with an electrically heated mixing head (W 50 EHT mixer) having 55 cm3 volume capacity and two noninterchangeable rotors. The processing temperature, rotor speed and blending time were set at 170 ◦ C, 60 rpm and 10 min, respectively. The sample volume of each blending was kept at 90% of volume capacity of mixer to study the variation in torque of composites of nano- and micro-fillers and for this purpose the total sample mass (62 g) was kept constant. The compounds of compos-
3. Results and discussion 3.1. Particle size analysis The size (nanoparticles) was determined from X-ray data using Scherer’s formula which states that the, particle size d (Å) = k/2 cos . k = order of reflection, = 1.542 Å, = diffraction angle and 2 = full width at half maximum (FWHM). The XRD pattern of calcium phosphate shows (Fig. 1) the size of calcium phosphate nanoparticles is 22 ± 5 nm. The calcium phosphate is prepared in situ of polymer (PEG) which acting as growth medium. It is actually matrix mediated growth and control technique or it is
Fig. 1. X-ray diffraction pattern of calcium phosphate nanoparticles.
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5 wt.% of filler. Fig. 3(a) shows completely exfoliated structure of PVC/calcium phosphate microcomposites, while Fig. 3(b) shows prominent sharp peaks at 2 = 25.6◦ for filler content 3 and 5 wt.%, which belongs to the calcium phosphate nanoparticles. The subsiding of peaks at 2 = 25.6◦ in Fig. 3(a) in the case of micro-calcium phosphate filled composites at all wt.% (1, 3 and 5) of fillers indicates fairly good dispersions. However, in the case of nano-calcium phosphate filled composites at 3 and 5 wt.% of filler content, the peaks do not subside rather they sharpens and the peak height at 2 = 25.6◦ increases with the increasing content of the filler (3 and 5 wt.%). Therefore it can be concluded that the structure of nanocomposites at 3 and 5 wt.% is intercalated and flocculated and not exfoliated. The subsiding of this emerging peaks at 2 = 25.6◦ suggests that nano-calcium phosphate did not mixed homogeneously at higher contents because of their high surface energy and strong tendency to form agglomerates. Again these results confirm that melt intercalation of nano-calcium phosphate and PVC at higher filler contents led to the phase separated system. 3.3. Morphology study
Fig. 2. TEM micrographs (a and b) of calcium phosphate nanoparticles prepared by in situ deposition technique.
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also called as in situ deposition technique. In in situ deposition technique firstly the calcium salt forms the complex with PEG polymer in which the calcium ions bound with cage of oxygen atom of the polymer. This is then reacted with ammonium phosphate during which the phosphate ions diffuse into the polymer. Since calcium ions are bound by polymer chains, the formation of calcium phosphate can take place at certain sites, which are dependent on the relative concentration of polymer. The distance between these sites depends on the PEG concentration with respect to the CaCl2 . The particle size of the calcium phosphate was found to be in the range of 47–97 nm, as confirmed from TEM micrographs (a and b) of Ca3 (PO4 )2 nanoparticles (Fig. 2).
In order to quantify the relative degree of dispersion of microand nano-calcium phosphate particles within the PVC matrix, tensile fractured surface of selected specimens was observed under SEM for their morphological studies. The micrographs for different samples are shown in Fig. 4(a–f). Fig. 4(a) and (d) for micro- and nanocomposites of calcium phosphate with 1% of filler, respectively, clearly reveal that the fracture surface is glossy, smooth and featureless, while Fig. 4(b) and (e) for 3% of filler shows fibrous matrix surface. However, in Fig. 4(c) and (f) for 5% of filler, the fracture surface of matrix is rough and contains some elliptical voids. From Fig. 4, it is clear that the dispersion of filler is good at 1% filler loading for both composites, while dispersion of microcomposites at 3 and 5% of filler possess few (hardly any one) agglomerates. The nano-calcium phosphate particle agglomerates severely at the 3 and 5 wt.% of filler in PVC matrix owing to their larger specific surface area and high polar surface energy. The aggregated calcium phosphate nanoparticles had poor compatibility with PVC matrix because of their hydrophilic surface, which led to cavities in the matrix and interface debond.
3.2. Wide angle X-ray diffraction studies (WAXD)
Fig. 3(a) and (b) shows WAXD patterns of PVC/micro- and nano-calcium phosphate composites, respectively, of 1, 3 and
3.4. Tensile behavior The tensile behavior of PVC/micro- and nano-Ca3 (PO4 )2 composites prepared by melt intercalation is shown in Fig. 5. The study on tensile behavior reflected that the tensile strength was decreased monotonously with the increasing filler content. However, the tensile strength of the nanocomposites with 1–4 wt.%
Fig. 3. WAXD of (a) PVC/micro-Ca3 (PO4 )2 for composites of 1, 3 and 5 wt.% of filler and (b) PVC/nano-Ca3 (PO4 )2 .
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Fig. 4. SEM micrograph: (a–c) PVC/micro-Ca3 (PO4 )2 composites and (d–f) PVC/nano-Ca3 (PO4 )2 composites at 1, 3 and 5 wt% of filler.
of filler content is higher than that of microcomposites and pristine PVC, while for 5 wt.% of filler it was found to be less than pristine PVC but slightly higher than microcomposites. It is known that the tensile strength of composites is influenced by filler fraction and the interfacial adhesion between fillers and the matrix. Due to tendency of agglomeration, the increasing addition of the nano-Ca3 (PO4 )2 particles, resulted in the existence of a weak interfacial adhesion
between the PVC matrix and the nanoparticles. Hence the load bearing capacity of cross-sectional area of composites decreased and only a small amount of stress could be transferred from the matrix to inorganic nanoparticles and thus, tensile strength showed decrement in the magnitude. In this case agglomerated particles easily debonded from the matrix and could not bear any fraction of external load ultimately decreasing the tensile strength. These results are inconsistent with the results of the research study carried out by Tianbin et al. [16], Sun et al. [5], and Jie et al. [24], Xiao-Lin et al. [6], Dongyan and Charles [34]. 3.5. Thermogravimetric analysis (TGA)
Fig. 5. Tensile strength of composites (micro and nano) with the variation of filler content (1–5 wt.%).
The representative TGA curves of PVC/micro- and nano-calcium phosphate composites are shown in Fig. 6 and their numerical data is summarized in Table 1. The information in this table includes the temperature at which 10% degradation occurs, a measure of onset of the thermal degradation; the temperature at which 50% degradation occurs, a measure of the midpoint of degradation and the fraction that is non-volatile at 550 ◦ C. Since the polymer matrix is same for all composites, thermal stability up to 300 ◦ C was common; however, the effect of filler was pronounced at temperature beyond 300 ◦ C that is, after first mechanistic step of degradation had taken place, this range was approximately 300–550 ◦ C. A measure-
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Table 1 TGA data of PVC/micro- and nano-Ca3 (PO4 )2 composites. Wt.% of filler
PVC/micro-Ca3 (PO4 )2 composites ◦
◦
10% mass loss ( C)
50% mass loss ( C)
Char at 550 C (%)
10% mass loss (◦ C)
50% mass loss (◦ C)
Char at 550 ◦ C (%)
266.4 269.0 268.6 270.8
293.0 292.0 294.6 301.0
09.0 12.0 09.0 05.0
266.4 268.3 268.3 268.5
293.0 296.0 287.5 278.0
09.0 05.0 06.5 08.5
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Pristine PVC 1 3 5
PVC/nano-Ca3 (PO4 )2 composites ◦
Fig. 8. Variation of tan ı with respect to temperature for pristine PVC and PVC/micro- and nano-Ca3 (PO4 )2 composites at 1, 3 and 5 wt.%.
Fig. 6. TGA curves of PVC/micro- and nano-Ca3 (PO4 )2 composites.
% of filler
Tg of PVC/micro-Ca3 (PO4 )2 composites (◦ C)
Tg of PVC/nano-Ca3 (PO4 )2 composites (◦ C)
0 1 3 5
91.2 91.7 91.7 91.3
91.2 93.6 91.7 91.4
From Fig. 7, it is clear that the E of PVC/micro-calcium phosphate composites at 1 and 3 wt.% was greater than that of pristine PVC, below the glass transition region, while at 5 wt.% it was less than pristine PVC in the same region. However, in case of nanocomposites the E of 1 wt.% nanocomposites was less than that of pristine PVC, however higher wt.% nanocomposites were having almost same E values. A similar trend was observed in the case of glass transition temperature obtained from tan ı versus temperature curve as onset of the curve of these composites. In general, the nanocomposites did not showed significant variation in storage modulus as well as glass transition temperature; rather the trend was mediocre compared to pristine PVC and microcomposites. Such unexpected viscoelastic behavior was again accounted for non-uniform distribution of nanoparticles and the formation of agglomerates causing nonhomogeneous interactions of organic matrix and the inorganic filler.
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ment of IPDT index allows us to compare thermal stability based on the area under the thermograms. Apparently the thermograms of PVC 1 and 3% nanocomposites are above the thermograms of pristine PVC and its corresponding microcomposites are in the range of 300–550 ◦ C. This indicates that IPDT values will be higher for 1 and 3% nanocomposites indicating that nano-calcium phosphate has favorable effect of enhancing thermal stability to some extent. The 10% mass loss values (Table 1) indicate that PVC/micro- and calcium phosphate composites exhibit marginal increment but definitely at higher sides as compared to pristine PVC. Besides this the non-volatile content is decreasing with increasing the filler content in case of both composites. As such it was expected to see sufficient increment in the values of 10% and 50% mass loss because of larger surface area of inorganic nanoparticles, however, this insignificant increment could be due to the agglomeration effect of nanoparticles as evident from the WAXD and SEM.
Table 2 Tg of PVC/micro- and nano-Ca3 (PO4 )2 composites.
3.6. Dynamic mechanical analysis
Dynamic storage modulus (E ) and tan ı as a function of temperature for pristine PVC and PVC/micro- and nano-Ca3 (PO4 )2 composites, are plotted in Figs. 7 and 8, respectively, and glass transition temperatures are summarized in Table 2.
Fig. 7. Variation of dynamic storage modulus (E ) with respect to temperature for pristine PVC and PVC/micro- and nano-Ca3 (PO4 )2 composites at 1, 3 and 5 wt.%.
4. Conclusion
Nano-Ca3 (PO4 )2 particles were successfully synthesized using in situ deposition approach, having size in the range of 47–97 nm, which was confirmed by TEM analysis. The PVC/micro- and nano-Ca3 (PO4 )2 composites were prepared by melt intercalation approach. The impact of nanoparticles on dispersion, thermal, dynamic mechanical, wide angle X-ray diffraction and tensile behavior was studied and compared with microcomposites. From the results of WAXD and SEM it was confirmed that the dispersion of filler was not proper in PVC matrix and hence the structure of PVC nanocomposites was intercalated and flocculated. The tensile strength of nanocomposites was found to be higher only at 1 wt.% filler content. No significant difference in dynamic storage modulus of both composites was observed in comparison with the pristine PVC. The glass transition temperature of both com-
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posites was observed slightly higher than pristine PVC. Further the onset degradation temperature of PVC/micro-Ca3 (PO4 )2 composites were marginally higher than nanocomposites and pristine PVC. The overall study on polymer composites, thus, reflected the poor interaction and dispersion of the Ca3 (PO4 )2 nanoparticles within the PVC matrix. Hence there is a need to modify interaction between PVC and nano-Ca3 (PO4 )2 by adapting other technique such as increasing shear force and or modifying surface of nanoparticles.
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Acknowledgement Authors thank the University Grants Commissions, New Delhi, for providing research grants under Special Assistance Programme (SAP) at Departmental Research Support (DRS) Level Grant No. F. 3-24/2004 (SAP-II), dated 15/10/2004 to the School of Chemical Sciences. References
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Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
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Preparation and characterization of poly(vinyl chloride) calcium phosphate nanocomposites Chetan B. Patil a,∗ , Priyanka S. Shisode a , Uday R. Kapadi b , Dilip G. Hundiwale b , Pramod P. Mahulikar b a b
S.S.V.P.S.L.K. Dr. P. R. Ghogrey Sciences College, Dhule, 424 001, M.S., India School of Chemical Sciences, North Maharashtra University, Jalgaon 425 001, M.S., India
a r t i c l e
i n f o
Article history: Received 2 August 2009 Received in revised form 24 November 2009 Accepted 6 December 2009
1. Introduction
Calcium phosphate [Ca3 (PO4 )2 ] was synthesized by in situ deposition technique and its nano-size (47–97 nm) was confirmed by Transmission Electron Microscopy (TEM). Composites of the filler Ca3 (PO4 )2 (micro and nano) and the matrix poly(vinyl chloride) (PVC) were prepared with different filler loadings (0–5 wt.%) by melt intercalation. The Brabender torque rheometer equipped with an internal mixer was used for the preparation of composites of different formulations. The effect of nano- and microCa3 (PO4 )2 content on the structure and properties of composites was studied. The nanostructures were studied by wide angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM). The mechanical, thermal and dynamic mechanical properties of PVC/micro- and nano-Ca3 (PO4 )2 composites were characterized using Universal Testing Machine (UTM), Thermo Gravimetric Analyzer (TGA) and dynamic mechanical analyzer (DMA). The thermal analysis results showed that the first thermal degradation onset (Tonset ) of PVC/nano-Ca3 (PO4 )2 composites was lower as compared with corresponding microcomposites and higher than that of pristine PVC. However, the tensile strength was found to be maximum at 1% of nano-Ca3 (PO4 )2 and again decreased with increasing loading of nano-Ca3 (PO4 )2 . Further storage modulus of PVC/micro- and nano-Ca3 (PO4 )2 composites was decreased with increasing loading while the glass transition temperature increased marginally. © 2009 Elsevier B.V. All rights reserved.
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Keywords: Poly(vinyl chloride) Calcium phosphate Nanoparticles Transmission Electron Microscopy (TEM) Wide angle X-ray diffraction (WAXD) Tensile strength Thermal stability (TGA), Dynamic mechanical properties (DMA)
a b s t r a c t
Polymer composites represent an important class of engineering materials. The incorporation of inorganic fillers into thermoplastics has been widely practiced in industry to extend them and to improve certain properties. One primary purpose of adding inorganic fillers to polymers is cost reduction. The use of fillers has been also a common practice to enhance the mechanical properties of thermoplastics, such as heat distortion temperature, hardness, toughness, dimension stability, stiffness, and mold shrinkage. The effects of filler on the mechanical and other properties of the composites depend strongly on filler origin, particle shape and size, aggregate size, the fraction of filler, surface characteristics and degree of dispersion. Most of the studies revealed that addition of rigid, inorganic fillers to polymers generally resulted in a significant decrease of toughness compared to neat polymers. There are, however, several studies demonstrating an increase in toughness with rigid particulate fillers in certain composite systems such as, for example, filled polypropylene (PP) and filled polyethylene
∗ Corresponding author. Tel.: +91 257 225 7434/9423777333; fax: +91 257 225 8403. E-mail address: [email protected] (C.B. Patil). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.12.034
(PE). An impressive increase of impact strength was reported by the literature for PE filled with calcium carbonate (CaCO3 ) particles [1–4]. In the recent decade, organic–inorganic nanocomposites have attracted a great interest to scientists, because nanocomposites have emerged as a very efficient strategy to upgrade properties of synthetic polymers to the point where performances exceed largely the ones of conventional composites. Nanocomposites could dramatically induce an improvement in mechanical and electrical properties, heat resistance, radiation resistance, and other properties as a result of the nanometric scale dispersion of the filler in the matrix. Different nanometric compounds such as CaCO3 , SiO2 , TiO2 and ZnO have been used to prepare the nanocomposites with various polymers, however, relatively little attention has been paid to poly(vinyl chloride) (PVC) materials [5–12]. PVC is an important commercial thermoplastic, has been studied and used widely in the industrial fields such as electrical insulators, plastics moldings, and building materials for many years. However, because of some inherent disadvantages, such as low thermal stability and brittleness, PVC and its composites are subjected to some limitations in certain applications. Recently, the development of polymer nanocomposites may present a new way to improve the performance of PVC [13–16]. When conventional toughening modifiers such as CPE, ACR, and NBR are replaced with nanoparticles
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2. Experimental 2.1. Materials
Calcium chloride, ammonium phosphate were of analytical grade and poly(ethylene glycol) (PEG; MW 6000 g) was procured from s. d. Fine Chem. Ltd., Mumbai, India and used for the synthesis of nanoparticles of calcium phosphate. The poly(vinyl chloride) (PVC) of grade (57GERO68) was obtained from Reliance Industries Ltd., Mumbai, India, as a matrix. Processing aid (PA 20), lubricant processing aid (LI 20), Glyceril monostearate as internal lubricant (LUB 11), PE wax as external lubricant and organotin stabilizer were obtained from Supreme Industries Ltd. Jalgaon, India. Calcium phosphate (5 m) is of commercial grade procured from s. d. Fine Chemicals, Company, Mumbai, India and was used as filler without any treatment. 2.2. Preparation of nanoparticles
ites were obtained in the form of lumps. These lumps of composites then crushed to the coarser particles (approximately 3–4 mm size) that were suitable for injection moulding to get tensile and impact testing specimens. 2.4. Characterization The particle size and morphology of the synthesized nanocalcium phosphate particles were studied using Transmission Electron Microscope (TEM, Philips Tecnai-20) at an accelerating voltage of 200 kV. Sample of calcium phosphate was dispersed in acetone with ultrasonic wave for sufficient 12 h before analysis for particle size. The structure of PVC–Ca3 (PO4 )2 nanocomposites was characterized by wide angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM). WAXD was performed on X’pert-Pro PANAlytical diffractometer (Philips) using Cu K␣ radiation at generated voltage of 40 kV and current of 30 mA at 25 ◦ C. The diffractions were scanned from 0◦ to 40◦ in the 2 range in 0.01◦ steps with a continuous scan. To study the dispersion of filler and morphology of composites, a tensile fractured surface was coated with platinum and observed on JEOL 6360 scanning electron microscope (SEM) at an acceleration voltage of 10 kV to obtain the scanning images. A thermogravimetric analyzer (TGA; Shimadzu, Japan, model: TGA-50/50H) was used to analyze thermal characteristics of the PVC/micro- and nano-Ca3 (PO4 )2 composites. The composites were heated from room temperature to 550 ◦ C at the rate of 5 ◦ C/min under nitrogen stream. The onset temperature and the weight loss during thermal degradation of composites were recorded and analyzed. Tensile strength was determined by subjecting dumb-bell shaped specimens (in accordance with ASTM D-638) to a Universal Testing Machine (UTM 2302, R & D Equipment, Mumbai, India). The specimens were conditioned for 24 h prior to testing. The load cell of 3000 kg and a crosshead speed of 50 mm/min were employed. The dynamic mechanical analysis (DMA) was performed using a dynamic mechanical analyzer (Gabo Eplexor, Swiss) at fixed frequency rate of 10 Hz in a temperature range from 30 to 120 ◦ C with a heating rate of 2 ◦ C/min.
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such as CaCO3 or Ca3 (PO4 )2 , the fracture behavior is expected to be changed [17–26]. In the present study, the nanoparticles of Ca3 (PO4 )2 , were synthesized by in situ deposition technique and their particles size was confirmed with TEM. The melt intercalation of PVC with nano- and commercial Ca3 (PO4 )2 were carried out using Brabender Plastograph EC, which gave data on rheological behavior. The composites so formed were tested for their physical, mechanical, thermal and structural properties and morphology of tensile fractured surfaces was also investigated [27–35].
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The nano-sized calcium phosphate filler particles were synthesized using in situ deposition technique which is actually matrix mediated growth and control technique or it is also called as in situ deposition technique. The calcium phosphate is prepared in situ of polymer (PEG) which acts as a growth medium as described below [9–11]. Calcium chloride (111 g, 1 m) was dissolved in distilled water (140 mL) and PEG (372 g, 0.062 m) was separately dissolved in hot distilled water (300 mL). The solutions were mixed properly and gently digested for 12 h. Solution of ammonium phosphate (115 g, 1 m) in distilled water (250 mL) was then added slowly with stirring to the complex and the nanoparticles formed were allowed to digest overnight. The nanoparticles formed were filtered (Whatman filter paper no. 41), washed thoroughly with distilled water, till freed from PEG traces. The prepared filler was then dried at 110 ◦ C for 2 h. Before compounding, the filler was heated at about 250 ◦ C for removing traces of moisture. 2.3. Preparation of composites
The PVC compounds were formulated with 2.5 phr of processing aid (PA 20), 2.5 phr of lubricating processing aid (LI 20), 3.5 phr of glyceril monostearate internal lubricant (LUB 11), 3.5 phr of organotin stabilizer, 2.0 phr of PE wax as external lubricant and fillers both nano- and micro-calcium phosphate with variable concentration (0–5%). These formulations were dry blended in domestic mixer for 15 min. For preparation of composites the pre-powder of these formulations was melt intercalated using Brabender Plastograph EC equipped with an electrically heated mixing head (W 50 EHT mixer) having 55 cm3 volume capacity and two noninterchangeable rotors. The processing temperature, rotor speed and blending time were set at 170 ◦ C, 60 rpm and 10 min, respectively. The sample volume of each blending was kept at 90% of volume capacity of mixer to study the variation in torque of composites of nano- and micro-fillers and for this purpose the total sample mass (62 g) was kept constant. The compounds of compos-
3. Results and discussion 3.1. Particle size analysis The size (nanoparticles) was determined from X-ray data using Scherer’s formula which states that the, particle size d (Å) = k/2 cos . k = order of reflection, = 1.542 Å, = diffraction angle and 2 = full width at half maximum (FWHM). The XRD pattern of calcium phosphate shows (Fig. 1) the size of calcium phosphate nanoparticles is 22 ± 5 nm. The calcium phosphate is prepared in situ of polymer (PEG) which acting as growth medium. It is actually matrix mediated growth and control technique or it is
Fig. 1. X-ray diffraction pattern of calcium phosphate nanoparticles.
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5 wt.% of filler. Fig. 3(a) shows completely exfoliated structure of PVC/calcium phosphate microcomposites, while Fig. 3(b) shows prominent sharp peaks at 2 = 25.6◦ for filler content 3 and 5 wt.%, which belongs to the calcium phosphate nanoparticles. The subsiding of peaks at 2 = 25.6◦ in Fig. 3(a) in the case of micro-calcium phosphate filled composites at all wt.% (1, 3 and 5) of fillers indicates fairly good dispersions. However, in the case of nano-calcium phosphate filled composites at 3 and 5 wt.% of filler content, the peaks do not subside rather they sharpens and the peak height at 2 = 25.6◦ increases with the increasing content of the filler (3 and 5 wt.%). Therefore it can be concluded that the structure of nanocomposites at 3 and 5 wt.% is intercalated and flocculated and not exfoliated. The subsiding of this emerging peaks at 2 = 25.6◦ suggests that nano-calcium phosphate did not mixed homogeneously at higher contents because of their high surface energy and strong tendency to form agglomerates. Again these results confirm that melt intercalation of nano-calcium phosphate and PVC at higher filler contents led to the phase separated system. 3.3. Morphology study
Fig. 2. TEM micrographs (a and b) of calcium phosphate nanoparticles prepared by in situ deposition technique.
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also called as in situ deposition technique. In in situ deposition technique firstly the calcium salt forms the complex with PEG polymer in which the calcium ions bound with cage of oxygen atom of the polymer. This is then reacted with ammonium phosphate during which the phosphate ions diffuse into the polymer. Since calcium ions are bound by polymer chains, the formation of calcium phosphate can take place at certain sites, which are dependent on the relative concentration of polymer. The distance between these sites depends on the PEG concentration with respect to the CaCl2 . The particle size of the calcium phosphate was found to be in the range of 47–97 nm, as confirmed from TEM micrographs (a and b) of Ca3 (PO4 )2 nanoparticles (Fig. 2).
In order to quantify the relative degree of dispersion of microand nano-calcium phosphate particles within the PVC matrix, tensile fractured surface of selected specimens was observed under SEM for their morphological studies. The micrographs for different samples are shown in Fig. 4(a–f). Fig. 4(a) and (d) for micro- and nanocomposites of calcium phosphate with 1% of filler, respectively, clearly reveal that the fracture surface is glossy, smooth and featureless, while Fig. 4(b) and (e) for 3% of filler shows fibrous matrix surface. However, in Fig. 4(c) and (f) for 5% of filler, the fracture surface of matrix is rough and contains some elliptical voids. From Fig. 4, it is clear that the dispersion of filler is good at 1% filler loading for both composites, while dispersion of microcomposites at 3 and 5% of filler possess few (hardly any one) agglomerates. The nano-calcium phosphate particle agglomerates severely at the 3 and 5 wt.% of filler in PVC matrix owing to their larger specific surface area and high polar surface energy. The aggregated calcium phosphate nanoparticles had poor compatibility with PVC matrix because of their hydrophilic surface, which led to cavities in the matrix and interface debond.
3.2. Wide angle X-ray diffraction studies (WAXD)
Fig. 3(a) and (b) shows WAXD patterns of PVC/micro- and nano-calcium phosphate composites, respectively, of 1, 3 and
3.4. Tensile behavior The tensile behavior of PVC/micro- and nano-Ca3 (PO4 )2 composites prepared by melt intercalation is shown in Fig. 5. The study on tensile behavior reflected that the tensile strength was decreased monotonously with the increasing filler content. However, the tensile strength of the nanocomposites with 1–4 wt.%
Fig. 3. WAXD of (a) PVC/micro-Ca3 (PO4 )2 for composites of 1, 3 and 5 wt.% of filler and (b) PVC/nano-Ca3 (PO4 )2 .
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Fig. 4. SEM micrograph: (a–c) PVC/micro-Ca3 (PO4 )2 composites and (d–f) PVC/nano-Ca3 (PO4 )2 composites at 1, 3 and 5 wt% of filler.
of filler content is higher than that of microcomposites and pristine PVC, while for 5 wt.% of filler it was found to be less than pristine PVC but slightly higher than microcomposites. It is known that the tensile strength of composites is influenced by filler fraction and the interfacial adhesion between fillers and the matrix. Due to tendency of agglomeration, the increasing addition of the nano-Ca3 (PO4 )2 particles, resulted in the existence of a weak interfacial adhesion
between the PVC matrix and the nanoparticles. Hence the load bearing capacity of cross-sectional area of composites decreased and only a small amount of stress could be transferred from the matrix to inorganic nanoparticles and thus, tensile strength showed decrement in the magnitude. In this case agglomerated particles easily debonded from the matrix and could not bear any fraction of external load ultimately decreasing the tensile strength. These results are inconsistent with the results of the research study carried out by Tianbin et al. [16], Sun et al. [5], and Jie et al. [24], Xiao-Lin et al. [6], Dongyan and Charles [34]. 3.5. Thermogravimetric analysis (TGA)
Fig. 5. Tensile strength of composites (micro and nano) with the variation of filler content (1–5 wt.%).
The representative TGA curves of PVC/micro- and nano-calcium phosphate composites are shown in Fig. 6 and their numerical data is summarized in Table 1. The information in this table includes the temperature at which 10% degradation occurs, a measure of onset of the thermal degradation; the temperature at which 50% degradation occurs, a measure of the midpoint of degradation and the fraction that is non-volatile at 550 ◦ C. Since the polymer matrix is same for all composites, thermal stability up to 300 ◦ C was common; however, the effect of filler was pronounced at temperature beyond 300 ◦ C that is, after first mechanistic step of degradation had taken place, this range was approximately 300–550 ◦ C. A measure-
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Table 1 TGA data of PVC/micro- and nano-Ca3 (PO4 )2 composites. Wt.% of filler
PVC/micro-Ca3 (PO4 )2 composites ◦
◦
10% mass loss ( C)
50% mass loss ( C)
Char at 550 C (%)
10% mass loss (◦ C)
50% mass loss (◦ C)
Char at 550 ◦ C (%)
266.4 269.0 268.6 270.8
293.0 292.0 294.6 301.0
09.0 12.0 09.0 05.0
266.4 268.3 268.3 268.5
293.0 296.0 287.5 278.0
09.0 05.0 06.5 08.5
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Pristine PVC 1 3 5
PVC/nano-Ca3 (PO4 )2 composites ◦
Fig. 8. Variation of tan ı with respect to temperature for pristine PVC and PVC/micro- and nano-Ca3 (PO4 )2 composites at 1, 3 and 5 wt.%.
Fig. 6. TGA curves of PVC/micro- and nano-Ca3 (PO4 )2 composites.
% of filler
Tg of PVC/micro-Ca3 (PO4 )2 composites (◦ C)
Tg of PVC/nano-Ca3 (PO4 )2 composites (◦ C)
0 1 3 5
91.2 91.7 91.7 91.3
91.2 93.6 91.7 91.4
From Fig. 7, it is clear that the E of PVC/micro-calcium phosphate composites at 1 and 3 wt.% was greater than that of pristine PVC, below the glass transition region, while at 5 wt.% it was less than pristine PVC in the same region. However, in case of nanocomposites the E of 1 wt.% nanocomposites was less than that of pristine PVC, however higher wt.% nanocomposites were having almost same E values. A similar trend was observed in the case of glass transition temperature obtained from tan ı versus temperature curve as onset of the curve of these composites. In general, the nanocomposites did not showed significant variation in storage modulus as well as glass transition temperature; rather the trend was mediocre compared to pristine PVC and microcomposites. Such unexpected viscoelastic behavior was again accounted for non-uniform distribution of nanoparticles and the formation of agglomerates causing nonhomogeneous interactions of organic matrix and the inorganic filler.
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ment of IPDT index allows us to compare thermal stability based on the area under the thermograms. Apparently the thermograms of PVC 1 and 3% nanocomposites are above the thermograms of pristine PVC and its corresponding microcomposites are in the range of 300–550 ◦ C. This indicates that IPDT values will be higher for 1 and 3% nanocomposites indicating that nano-calcium phosphate has favorable effect of enhancing thermal stability to some extent. The 10% mass loss values (Table 1) indicate that PVC/micro- and calcium phosphate composites exhibit marginal increment but definitely at higher sides as compared to pristine PVC. Besides this the non-volatile content is decreasing with increasing the filler content in case of both composites. As such it was expected to see sufficient increment in the values of 10% and 50% mass loss because of larger surface area of inorganic nanoparticles, however, this insignificant increment could be due to the agglomeration effect of nanoparticles as evident from the WAXD and SEM.
Table 2 Tg of PVC/micro- and nano-Ca3 (PO4 )2 composites.
3.6. Dynamic mechanical analysis
Dynamic storage modulus (E ) and tan ı as a function of temperature for pristine PVC and PVC/micro- and nano-Ca3 (PO4 )2 composites, are plotted in Figs. 7 and 8, respectively, and glass transition temperatures are summarized in Table 2.
Fig. 7. Variation of dynamic storage modulus (E ) with respect to temperature for pristine PVC and PVC/micro- and nano-Ca3 (PO4 )2 composites at 1, 3 and 5 wt.%.
4. Conclusion
Nano-Ca3 (PO4 )2 particles were successfully synthesized using in situ deposition approach, having size in the range of 47–97 nm, which was confirmed by TEM analysis. The PVC/micro- and nano-Ca3 (PO4 )2 composites were prepared by melt intercalation approach. The impact of nanoparticles on dispersion, thermal, dynamic mechanical, wide angle X-ray diffraction and tensile behavior was studied and compared with microcomposites. From the results of WAXD and SEM it was confirmed that the dispersion of filler was not proper in PVC matrix and hence the structure of PVC nanocomposites was intercalated and flocculated. The tensile strength of nanocomposites was found to be higher only at 1 wt.% filler content. No significant difference in dynamic storage modulus of both composites was observed in comparison with the pristine PVC. The glass transition temperature of both com-
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posites was observed slightly higher than pristine PVC. Further the onset degradation temperature of PVC/micro-Ca3 (PO4 )2 composites were marginally higher than nanocomposites and pristine PVC. The overall study on polymer composites, thus, reflected the poor interaction and dispersion of the Ca3 (PO4 )2 nanoparticles within the PVC matrix. Hence there is a need to modify interaction between PVC and nano-Ca3 (PO4 )2 by adapting other technique such as increasing shear force and or modifying surface of nanoparticles.
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Acknowledgement Authors thank the University Grants Commissions, New Delhi, for providing research grants under Special Assistance Programme (SAP) at Departmental Research Support (DRS) Level Grant No. F. 3-24/2004 (SAP-II), dated 15/10/2004 to the School of Chemical Sciences. References
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