Effect of addition of hydroxyapatite as secondary filler in CNT-reinforced polypropylene hybrid composites

Abstract

This work entails the effectiveness of hydroxyapatite (HAp) as secondary filler in polypropylene (PP) matrix composites reinforced with HAp-decorated multiwalled carbon nanotubes (MWCNTs). Different composites were prepared through the melt mixing approach. The consequent effect of typical biocompatible ceramic coating on the dispersion of CNTs in a PP matrix and thereby effect on its thermal and mechanical properties have been discussed in this research work. Isothermal hydroxylation technique has been used for the coating of HAp on MWCNTs. It has been observed from the collective information obtained from X-ray diffraction, Fourier-transform infrared spectroscopy, Raman spectroscopy and field-emission scanning electron microscopy images that HAp constitutes a new phase in the HAp-CNT nanocomposite in the form of a coating of HAp over nanotubes. As a result of which, the use of HAp-coated MWCNTs in PP improved its tensile strength by 38%, modulus of elasticity by 72%, storage modulus by 43% and ductility by around 7.4% in comparison to uncoated MWCNT-reinforced PP composite. However, the coating of HAp shows an insignificant effect for change in temperature-dependent properties of the CNT/PP composites. The improvement in mechanical properties is attributed to the interfacial compatibility between HAp and PP.

Keywords

Polypropylene hydroxyapatite multiwalled carbon nanotubes thermomechanical properties

Introduction

Carbon nanotubes (CNTs) are one of the most efficient carbon-based nanomaterials which are popular due to their ultra-high strength and conductive nature. These properties made them promising nanofiller for polymer matrix composites.^1-4 Various researchers have claimed that the addition of CNTs in different polymers extends the applicability of such composites for electromagnetic interference shielding, antistatic coatings, structural and corrosion-resistant components, energy storage materials, and so on.^3,4 The ease of processing and low cost of thermoplastics are two important factors which invited researchers to explore the possibilities to extend their use in the synthesis of advanced composites. Furthermore, the choice of thermoplastics, depending upon the application, may invoke the amorphous or semi-crystalline nature which mainly affects their behaviour during heating or thermo-processing. Polypropylene (PP) is one of the semi-crystalline thermoplastic polymers which can be popularly used in packaging industries, laboratory equipment, selective automotive components and various general-purpose applications.^5-8 Some typical characteristics of PP which differentiate it among other thermoplastics include its superior mechanical properties (in terms of stiffness, low density and high fatigue resistance) as well as high thermal and chemical stability.^9,10 On the other hand, the average strength and thermomechanical behaviour of such polymers is the main issue which invokes further opportunities to make it more appropriate for the above-mentioned applications. In the past many years, various researchers have exposed that the addition of selective micro- and nanofillers in such polymer matrices is worthy to overcome these limitations up to a significant extent.^11,12

Carbon-based nanofillers have shown tremendous potential for improvement in the typical mechanical, thermal, barrier and electrical properties of such polymer matrix composites. CNTs are considered to be superior to conventional micro-fillers due to their lower percolation threshold required for preparing a conductive network in various amorphous polymer composites. However, the large-scale production of good-quality multiwalled carbon nanotubes (MWCNTs) is even more challenging than the production of graphene nanosheets. The use of CNT for the synthesis of different polymer composites has already been claimed in various research articles where authors have reported significant improvement in conductivity and/or strength of the composites.^13-16 Particularly, CNT comprises high Young’s modulus of around 1 TPa and tensile strength around 11–63 GPa which makes them a distinguished nanoscale ultra-high-strength reinforcing material for polymer matrices.^17,18 However, there are certain limitations of using pure CNT as fillers in organic polymer matrices which arises the need for their surface modification or use of secondary filler to improve the effectiveness of reinforcing nanomaterial in hybrid composites.¹⁹

Hydroxyapatite (HAp) is a biocompatible ceramic which consists of typical components of teeth and human bones. It includes citrate and the combination of calcium with phosphate, carbonate, fluoride and hydroxide. In recent years, a few related research works have been focused to prepare HAp-CNT composites for improving the dispersibility of CNT in ceramic matrices.^20,21 In addition, Wang et al.²² presented that the tensile and impact strength of HAp-loaded PP is higher than neat PP. Li and Tjong²³ demonstrated that the addition of nanosized HAp into PP leads to an increase in the tensile modulus of nearly 25%. Some important findings using HAp as reinforcing element in typical polymer matrices have been provided in Table 1.

Table 1.

Effect of addition of HAp on mechanical properties of different polymers.

Composition	Significant effect	References
HAp-polyethylene	Increase in Young’s modulus up to 607% and tensile strength up to 26% with 40 vol% addition of HAp.	²⁴
HAp-ultra-high molecular weight polyethylene	Increase in Young’s modulus by nine times (i.e. 0.9–8.0 GPa, with 23 vol% addition of HAp).	²⁵
HAp-polyether ether ketone	Increase in Young’s modulus (4.3–11.4 GPa), compressive strength (113–139 MPa) and micro indentation hardness (12–23.82 VHN) with the addition of 20 vol% HAp.	²⁶
HAp-Al₂O₃-high-density polyethylene	High elastic modulus (6.2 GPa), low coefficient of friction (0.07–0.11), higher wear resistance (order of 10⁻⁶ mm³ Nm⁻¹) for HDPE-20 vol%-HAp-20 vol% Al₂O₃ composite.	²⁷
HAp-polysulphone and HAp-poly(lactide-co-glycolide)	Reduction of maximum stress generated in biostable composites for healing time of 6 weeks from 19.9 MPa to 5.7 MPa (for HAp-PSU) and 2.5 MPa to 1.9 MPa (for HAp-PGLA).	²⁸

HAp: hydroxyapatite; HDPE: high-density polyethylene; PSU: polysulfone; PGLA: poly(lactic-co-glycolic acid).

It can be observed from Table 1 that the addition of HAp resulted in improvements in mechanical properties of typical polymer composites. Although a common factor for a noticeable improvement in properties is not indicated, the microstructural and morphological discussions revealed information regarding good interfacial adhesion of added HAp particles with polymer surface. Although the mechanical properties of HAp are far less than corresponding properties of popular nanomaterials used for improvement in mechanical properties of the polymeric composites, it could be assumed that the addition of HAp particles acts as a hindrance to the progressing failure of polymer chains, due to comparative high strength. The addition of HAp has also shown improvement in distribution behaviour of Al₂O₃ in high-density polyethylene.²⁷ It suggests that the addition of HAp as secondary filler may also help for interaction between some inorganic nanomaterials and amorphous polymers. Therefore, this research work has been focused to study the effectiveness of HAp as a secondary filler and in the form of coating over CNTs. The effectiveness is considered to be dependent upon typical imperceptible parameters, that is, dispersion and interfacial adhesion between CNT and PP. These factors have been correlated based on tensile testing, thermal decomposition behaviour and thermomechanical properties of different composites.

Experimental procedure

Materials used and sample preparation

MWCNTs having 95% purity were purchased from Hanwha Nanotech Corporation (Republic of Korea). Phosphorus pentoxide (P₂O₅) having 97% purity was purchased from Loba Chemie Pvt. Ltd (Mumbai, Maharashtra, India). Ethanol (C₂H₆O) having 99% purity was purchased from Changshu Yangyuan Chemical Corporation Ltd (China). Calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O) having 99% purity, 85% pure sodium dodecyl sulphate and other chemicals or apparatus used for the preparation of HAp-CNT nanocomposite was purchased from HIMEDIA Laboratories Pvt. Ltd (Mumbai, Maharashtra, India).

Synthesis of HAp-CNT and hybrid composites

The coating of HAp over the surface of nanotubes has been done based on some previous works.^29,30 The main objective of slight changes in the existing methods was to achieve the desired coating properties and quality with the available resources. Herein, two different precursors were for preparation of intermediate solutions. Initially, 0.5 mol l⁻¹ solution was prepared by dissolving 28.39 g of P₂O₅ in 200 ml of absolute C₂H₆O. Then another solution of composition 1.67 mol l⁻¹ has been prepared by addition of 39.43 g of Ca(NO₃)₂·4H₂O, dropwise in 100 ml C₂H₆O. Both these solutions were then mixed to obtain the desired Ca/P molar ratio of 1.67. Thereafter, the solution was stirred at a speed of 650 r min⁻¹ at room temperature for 10–15 h or until the formation of a gel. After HAp gel preparation (milky colour solution), 100 mg CNT was added with 0.05 g sodium dodecyl sulfate in 25 ml H₂O and stirred for 1 h at room temperature. Then the prepared solution was added into the HAp gel solution. After that, the mixed solution was stirred for 90 min and filtered for drying. The prepared sample was dried in a vacuum oven at 50°C for 48 h. Finally, the samples were prepared in powdered form.

Different composites, as well as the processed PP specimen, were prepared via melt mixing approach using an internal mixer. In this process, the polymer pellets are initially added to the internal mixer which is maintained at a particular temperature (i.e. 180°C) and rotating at a particular speed (30 r min⁻¹). As the polymer pellets start deforming, filler is added to the chamber of an internal mixer and it is allowed to properly mix with the polymer until it transforms to a semi-solid form. At this instance, the semi-solid composite specimen is taken out of the internal mixer and moulded into desired shaped specimens under a load of 3 tonne which has been applied for about 10 min at a constant temperature of 120°C with the help of compression moulding machine. The moulded specimen is kept under the same pressure maintained with the help of compression moulding and allowed to cool to the room temperature slowly. Table 2 presents the composition of different samples used in this work.

Table 2.

Composition and tensile properties of different composites.

Sample codes	Composition (wt%)			Elastic modulus (GPa)	Tensile strength (MPa)	Elongation at break (%)
Sample codes	PP	CNT	HAp-CNT	Elastic modulus (GPa)	Tensile strength (MPa)	Elongation at break (%)
PP	100	–	–	0.9 ± 0.04	26.3 ± 0.6	86 ± 2.5
CNT/PP	95	5	–	1.1 ± 0.03	30.5 ± 1.2	95 ± 0.8
HAp-CNT/PP	95	–	5	1.9 ± 0.03	42.2 ± 0.9	102 ± 1.8

PP: polypropylene; CNT: carbon nanotubes; HAp: hydroxyapatite.

Instruments and characterisations

Bruker AXS diffractometer D8 (Bruker India Scientific Pvt. Ltd., Anandapur, Kolkata, India) having Cu Kα radiation, range of diffraction angle from 10° to 60° and scanning rate of 1 min⁻¹ was used to record the X-ray diffraction (XRD) patterns of nanomaterials. TGA SII 6300 Exstar instrument (LabX, Midland, Ontario, Canada) was used to record the thermal decomposition behaviour of the nanomaterials. Thermo Scientific Nicolet 6700 Fourier-transform infrared (FTIR) spectrometer (Invitrogen BioServices India Pvt. Ltd, Bangalore, Karnataka, India) was used to perform FTIR spectroscopy of the nanomaterials. An argon ion laser fitted Invia Renishaw Raman spectrophotometer (Renishaw Metrology Systems Ltd., Gurgaon, India) was used to record the Raman spectra of nanomaterials under the excitation wavelength of 514 nm. The morphological images of synthesised nanomaterials were recorded with the help of field-emission scanning electron microscopy (FESEM; Zeiss-Ultra Plus, Carl Zeiss India, Bangalore, Karnataka, India).

A universal tensile testing machine (Instron 5582, Applus DatapointLabs, Ithaca, New York, USA) was used to analyse the tensile properties of PP and its composites, at room temperature 25 ± 2°C. All the tests were performed under crosshead speed of 2 mm min⁻¹ and the composite specimen was prepared according to ASTM D638-10. The tensile properties have been obtained by testing five specimens for each composition. The viscoelastic properties of polymeric composites were analysed with the help of TA-Q800 model (Waters India Private Limited, Bengaluru, Karnataka, India) of dynamic mechanical analyzer (DMA) machine which was operated in dual cantilever mode at a constant frequency of 1 Hz. The temperature was varied from −75°C to 175°C under a heating rate of 3°C min⁻¹. The approximate dimension of a specimen prepared for DMA testing was 35.0 mm in length × 12.0 mm in width × 2.0 mm in thickness.

Results and discussions

Structure and morphology of nanomaterials

Figure 1 presents the effect of the addition of HAp on some basic properties of MWCNTs. Figure 1(a) shows the XRD diffractograms of as-received CNT and HAp-CNT nanocomposite. It is clear from these diffractograms that the intensity of (002) peak decreases after the addition of HAp. This resembles that the presence of HAp may be in the form of layered coating over the nanotubes which decreases the crystallinity of HAp-CNT nanocomposite. However, other characteristic peaks of HAp (211), (112) and (300) at 31.9°, 32.1° and 32.9°, respectively, cannot be observed because the intensity of these peaks is much lower than the intensity of (002) peak present at 26°. Figure 1(b) shows the thermal decomposition behaviour of CNT and HAp-CNT nanocomposite. It can be realised from the thermographs of these nanomaterials that the weight loss of HAp-CNT nanocomposite starts as the temperature rises above room temperature while that of pure CNT remained nearly unaffected up to 550°C. The gradual weight loss of HAp-CNT nanocomposite should be due to the partial dehydroxylation. Still, the residual weight of HAp-CNT nanocomposite is much higher than that of CNT. This ensures better thermal stability of HAp in comparison to pristine MWCNT. Furthermore, FTIR spectroscopy has been conducted to confirm the presence of HAp in HAp-CNT nanocomposite. Figure 1(c) shows two additional bands in HAp-CNT nanocomposite at 560 and 1110 cm⁻¹ which are attributed to the P–O bending of a phosphate group and O–P–O phosphate ions of hydroxyl site, respectively. Other common bands around 3452 and 1389 cm⁻¹ are attributed to the decrease in intensity of –OH deformation vibrations peaks and the band around 1637 cm⁻¹ represents C–C stretching of CNTs. The shifts in peaks at 560 and 1110 cm⁻¹ justify the coating of HAp over the surface of MWCNTs. The shift in wavenumbers is mainly attributed to the large ionic radius of Sr²⁺ present in apatite. Moreover, the presence of peak associated with 1389 cm⁻¹ indicates a good interaction among the surface of nanotubes and minerals present in HAp. Raman spectra of CNT and HAp-CNT have been shown in Figure 1(d). In addition to the D-band around 1350 cm⁻¹, G-band around 1580 cm⁻¹ and 2D-band around 2700 cm⁻¹, the Raman spectrum of HAp-CNT nanocomposite also contains a phosphate band around 915 cm⁻¹ which denotes characteristic of carbonated apatites, B-type carbonate band around 1015 cm⁻¹ related to bone mineral, followed by the component of a phosphate band around 1125 cm⁻¹. The peaks of D-band, G-band and 2D-band have also been shifted to slightly higher wavelengths. These observations indicate that the addition of HAp through adopted method don’t constitute a new phase and must exist as a layered coating over the surface of MWCNT.

Figure 1.

(a) XRD, (b) TGA, (c) FTIR and (d) Raman spectra of CNT and HAp-CNT nanocomposite.

Figure 2 shows the morphology and composition of the nanomaterials with the help of FESEM images and energy dispersive X-ray analysis (EDX) technique. The FESEM images represent the distribution behaviour of pristine nanotubes and also confirm the uniformity in diameter of nanotubes with significant aspect ratio. Further, the morphology shown in Figure 2(b) indicates that the addition of HAp improved the tendency for surface linearity and self-alignment of HAp-CNT nanocomposite without damaging the surface of nanotubes. The comparative results of EDX analysis ensure the presence of oxygen, phosphorus and calcium in HAp-CNT which are the constructive elements of calcium apatite. This is another evidence for the successful coating of HAp on CNT through the adopted methodology.

Figure 2.

Morphology and chemical composition of (a) CNT and (b) HAp-CNT nanocomposite observed using FESEM and EDX analysis.

Morphology of the composites

Figure 3 presents the surface morphology of PP and its composites with the help of FESEM micrographs. The smooth surface of PP shown in Figure 3(a) and (b) ensures that the air bubbles were not trapped during the melt mixing at specified conditions. Some small pores have been seen on the surface of PP in Figure 3(a). A close observation of these pores indicated their presence as small impressions on the surface due to the presence of unwanted small particles between the hot plate and the polytetrafluoroethylene sheet. The morphology of the surface of nanomaterial-reinforced PP composites obtained after the tensile fracture has been shown in Figure 3(c) and (d). The bright spots in these micrographs represent some standing rods which must be the detached reinforced nanotubes resulting shear failure during tensile loading between the surface of nanotubes and the polymer. Similar surface morphology of CNT/PP composites has been observed in some other research works.^31,32 Figure 3(e) and (f) indicates that the addition of HAp-CNT in the PP matrix brings better uniformity for dispersion of nanomaterial within the polymer matrix. In comparison to CNT/PP composites, the surface of nanotubes present in the cross-section of tensile fractured HAp-CNT/PP composites is conical. This indicates that the tensile fracture of HAp-CNT/PP composites is not governed by the shear failure between nanotubes and polymer matrix, as in the case of CNT/PP composites. The improved adhesion/bonding between nanotubes and PP should be accredited to the presence of carbonated apatites due to the coating of HAp over MWCNTs. Both these factors should be responsible for an increase in the resistance of hybrid composite to tensile failure.

Figure 3.

Morphology of (a and b) PP, (c and d) CNT/PP composite and (e and f) HAp-CNT/PP hybrid composite.

Mechanical and thermal properties of the composites

Table 2 shows the tensile properties of PP and unmodified/modified nanotube-loaded PP matrix composites. The observed values show 22% improvement in tensile modulus, 16% improvement in tensile strength and 10% improvement of ductility of PP with an addition of 5% MWCNTs. However, the addition of HAp-CNT in PP improves these properties by 111%, 60% and 19%, respectively. Corresponding change in tensile properties validates the presumed behaviour discussed based on the morphological images of the composites, shown in Figure 3(e) and (f). The linearity in the dispersion of MWCNTs forms a network inside the polymer matrix due to which the tendency for shear failure during the tensile testing decreases significantly. The simultaneous improvement in ductility and tensile strength shows that the coating of HAp over nanotubes significantly improves the mechanical stability of such hybrid composite.

Figure 4 shows the thermomechanical behaviour of pure polymer and different composites with the help of curves representing a change in storage modulus under increasing temperature and corresponding variation in the damping factor of the composites. Particularly in case of PP, these curves comprise three relaxations (i.e. α-relaxation, β-relaxation and γ-relaxation) at around −70°C, 8°C and 100°C, respectively. The tan δ peak corresponding to γ-relaxation particularly indicates the relaxation behaviour of amorphous polymer chains, β-relaxation peak indicates the glass transition region and the α-relaxation peak indicates the dislocation in crystalline phase through slip or twinning mode. In comparison to β- and γ-relaxation peaks, the α-relaxation peak is generally not recognised in the tan δ curve due to its low slope. The noticeable change in thermomechanical behaviour shown in Figure 4 corroborates the results obtained from the tensile test of the corresponding specimen. Particularly, 5 wt% addition of uncoated and HAp-coated CNT improved the storage modulus of the polymer at −75°C by 35% (3650 MPa for PP to 4950 MPa for CNT/PP composite) and 92% (3650 MPa for PP to 7030 MPa for HAp-CNT/PP composite), respectively. A close observation of the thermographs depicts that HAp-CNTs are effective than pristine CNTs for improvement in storage modulus of PP in both glassy and rubbery states. Furthermore, tan δ versus temperature curve of PP shows two transition peaks at around 15°C and 110°C. Herein, the peak at lower temperature represents the glass transition temperature (T_g) of polymer and another peak at higher temperature indicates the softening before fusion of the polymer. The comparative behaviour of curves representing damping behaviour of the composites suggests an insignificant effect of reinforcing components on T_g of the composites. An improvement in tensile properties in combination with similar effects in viscoelastic properties is an indication of the improved ability of CNT/PP composites to withstand under dynamic loading conditions due to coating of HAp over the surface of MWCNTs.

Figure 4.

Thermomechanical properties of different composites.

The thermal decomposition behaviour of PP, uncoated MWCNT-reinforced composites and HAp-coated MWCNT-reinforced composites have been shown in Figure 5. The behaviour of thermogravimetric curves indicates two important factors. First is that the loss of weight up to 250°C is negligible (i.e. no considerable weight loss) which represents the absence of moisture in the synthesised composite specimen. Another is that the total weight loss occurs in a single step under a varying rate of decomposition which represents the failure of polymer chains. The thermal decomposition of HAp-CNT/PP occurs at about 20°C higher than CNT/PP composites and 30°C higher than unreinforced PP. This shows an insignificant contribution of HAp coating or the addition of nanotubes for improvement in thermal stability of the PP matrix composites.

Figure 5.

Thermogravimetric curves of pure polymer and reinforced composites.

The collective mechanical and thermal analysis of the composites indicates that the coating of HAp improves the dispersion as well as interfacial bonding between MWCNTs and PP. It has already been discussed in the ‘Structure and morphology of nanomaterials’ and ‘Morphology of the composites’ sections that the HAp-coated MWCNTs consist of carbonate and acid phosphate which result in the formation of carbonated apatites. Herein, the presence of phosphate groups is mainly responsible for the homogeneous distribution of nanotubes in the PP matrix and carbonates assist for better interaction between the nanotubes and the polymer chains. The change in interfacial behaviour of nanotubes and polymer assisted for significant improvement in tensile and viscoelastic properties of the hybrid polymer composite.

Conclusions

This research article explores the effect of the addition of HAp, in the form of coating over the carbon-based nanotubes, on typical thermal and mechanical properties of the PP matrix composites. The successful coating of HAp over the surface of nanotubes has been confirmed through characteristic structural and morphological analysis techniques. Particularly, morphological images of the reinforced composites have shown that the coating of HAp changes the dispersion behaviour of MWCNTs in PP matrix which brings on a change in tensile and thermomechanical properties of the MWCNTs/PP composites. Typical mechanical and thermal characterisations have confirmed that the coating of HAp over MWCNTs has the significant ability for simultaneous improvement in desirable thermomechanical properties of nanotube-reinforced composites.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Kaushik Pal

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