- Email: [email protected]
Processing of hydroxyapatite reinforced ultrahigh molecular weight polyethylene for biomedical applications
ARTICLE IN PRESS
Biomaterials 26 (2005) 3471–3478 www.elsevier.com/locate/biomaterials
Processing of hydroxyapatite reinforced ultrahigh molecular weight polyethylene for biomedical applications Liming Fanga, Yang Lenga,, Ping Gaob a
Department of Mechanical Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China b Department of Chemical Engineering, The Hong Kong University of Science & Technology, Hong Kong, China Received 6 July 2004; accepted 8 September 2004
Abstract A new method for processing hydroxyapatite/ultrahigh molecular weight polyethylene (HA/UHMWPE) composite has been developed by combining wet ball milling and swelling. Sintered HA particles were ground in ethanol to 50 nm in diameter. The nano-sized HA particles were mechanically mixed with UHMWPE in the ball mill and then compression molded into solid slabs. The slabs were then swollen in a pharmaceutical grade paraffin oil to enhance the UHMWPE chain mobility and HA/UHMWPE interface adhesion before final hot press. The resultant composite exhibits a two-zone network structure formed by a homogeneous HA-rich phase and a UHMWPE-rich phase. This process resulted in a 90% increase in Young’s modulus and a 50% increase in the yield strength of HA/UHMWPE composite, comparing with those of unfilled UHMWPE. r 2004 Elsevier Ltd. All rights reserved. Keywords: Hydroxyapatite; UHMWPE; Composite; Swelling; Ball milling
1. Introduction Since Bonfield et al. [1] pioneered the use of hydroxyapatite (HA) to reinforce high density polyethylene (HDPE) for bone substitutes, there has been much interest in developing bio-analogue composites that have advantages over conventional artificial hardtissue replacement materials [2–3]. The mechanical and biological properties of bio-analogue composites can be tailored to meet specific clinical requirements [4]. Although HDPE has been widely used as the matrix for bio-composites [1,5–12], its wear resistance and impact strength are inferior to ultrahigh molecular weight polyethylene (UHMWPE) [13]. The use of UHMWPE as a composite matrix has been explored with various fillers, such as carbon (carbon black [14], carbon fiber [15], carbon nanotube [16], graphite [15]), ceramics (kaolin [17], wollastonite [18], Corresponding author. Tel.: +852 2358 7185; fax: +852 2358 1543.
E-mail address: [email protected] (Y. Leng). 0142-9612/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biomaterials.2004.09.022
quartz [19], TiO2 [20], A12O3 [21], CaCO3 [22], etc), or metals [23], etc. Generally, methods for processing UHMWPE composites include solid-state mixing and thermal forming [14–15,18,20–23], solution gelation and crystallization [16,19], and in situ polymerization [17]. Solid-state mixing and thermal forming process is easy to be performed but the filler/matrix interfacial strength is poor because only interaction between the filler and the matrix is mechanical interlocking, even though chemical surface treatment is employed [15,18,21–22]. Solution gelation and crystallization method is mainly utilized to produce an electrical conductive polymer blended with a very low fraction of carbon, but the mechanical properties are not the major concern. In situ polymerization of a UHMWPE composite requires strict processing conditions and has the problem of catalyst removal, which has only been tried for Kaolin/ UHMWPE composite [17]. Several researches on HA/UHMWPE composite have also been reported. Knets et al. [24] produced an annealed compact bone tissue reinforced UHMWPE
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composite, but few processing details were given. Cunha et al. [25] studied the influence of processing conditions on the mechanical behavior of the HA/UHMWPE composite using injection and compression molding, and found that injection moulding process assured better results than the compression molding due to the intensive mixing and shearing of the polymer. Reis et al. [26] investigated the impact behavior of the HA/UHMWPE composite. The polymer they used ¯ w ¼ 225; 000 g=mol; BP was Rigidex HM4560XP (M Chemicals Ltd., UK) [6], which actually is a type of HDPE. Use of UHMWPE as a composite matrix, however, has not achieved much success due to difficulties in processing. Its extremely high molecular weight makes it unprocessable by conventional thermoplastic processing techniques [27]. Furthermore, dispersion of HA particles in UHMWPE has been a serious challenge because of the latter’s extremely high viscosity [28], and UHMWPE is prone to fusion defects due to the long relaxation time of UHMWPE chains [29]. Therefore, it is necessary to develop new processing methods to fabricate a biocomposite with homogenous HA particles distribution in UHMWPE matrix and with interpenetration between HA particles and UHMWPE chains. In this study, we developed a novel method to process the HA/ UHMWPE composite by wet ball-mill compounding and swelling in a non-toxic solvent.
Fig. 1. SEM micrograph of as-received UHMWPE particle.
2. Materials and experimental methods The UHMWPE was the HiFax 1900 reactor powder (Basell Ltd, USA) with a molecular weight of 6 106 g/ mol and an average size of 200 mm (Fig. 1). The HA particles were the product of wet synthesis and sintering at 900 1C for 2 h (Fig. 2), provided by the Engineering Research Center for Biomaterials at Sichuan University, China. To investigate the effects of ball milling and swelling, four HA/UHMWPE samples were prepared using different combinations of processing techniques: (1) as-received HA reinforced UHMWPE unswollen treated; (2) ball-milled HA reinforced UHMWPE unswollen treated; (3) as-received HA reinforced UHMWPE swollen treated; and (4) ball-milled HA reinforced UHMWPE swollen treated. Unfilled UHMWPE specimens were also prepared in a similar manner for comparison. The ball milling was conducted in a rotary ball mill (PascallTM, Pascall Engineering Co. Ltd, Crawley, UK) with adjustable rotation speed up to 3000 rpm. The mill container volume was 1.5 L and the stainless steel ball diameter was 12.3 mm. The mill container and balls were cleaned by rinsing in an analytical grade ethanol (99.8%, Merck, Germany) before ball milling. In processing of sample 2 and 4, HA particles were ball
Fig. 2. SEM micrograph of as-received HA particles.
milled in ethanol for 8 h followed by drying in a vacuum oven at 80 1C overnight. And then the ball-milled HA powder (30 wt%) was compounded with UHMWPE reactor powder also by ball milling in ethanol for 4 h. Subsequently, the mixture was dried in a vacuum oven at 80 1C for 24 h. Afterwards, it was mechanically compacted into slabs of 38 mm in diameter and 1 mm in thickness at 135 1C. The swelling treatment of the as-compacted slab was carried out in pharmaceutical grade paraffin oil (Merck, Germany). The slab was immersed in paraffin oil at 135 1C for 10 h to reach a swelling ratio of 10. Then, the oil was removed by hot press (TMPs, Technical
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Machine Products, Ohio, USA) at 100 1C and further extracted by hexane (analytical grade, Merck, Germany) for 24 h. After extraction, the slab was air-dried overnight to allow the excess solvent to evaporate. Finally, the slab was hot pressed under 100 MPa at 180 1C to form HA/UHMWPE composite sheets. The size of HA particles was analyzed with Coulter LS230 (Beckman-Coulter, Florida, USA). The microstructure of the HA/UHMWPE composite was characterized using SEM (JEOL 6300, JEOL, Tokyo, Japan) and TEM (Philips CM 20, Philips Analytic, The Netherlands). The TEM samples were prepared by embedding a small piece of composite in epoxy and sectioning the embedded samples using ultracut-R ultramicrotome (Leica, Bensheim, Germany) with a diamond knife. The volume fraction of HA in the composite was measured from SEM micrographs by an image analyzer (Qwin Pro 2.2, Leica). Mechanical
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properties were evaluated by a universal testing machine (Instron 5500, Instron Co., Massachusetts, USA) with a load cell of 100 N. Tensile specimens were 50 mm long and 5 mm wide with a gauge length of 20 mm, which was approximately 0.2 mm thick. Six specimens were tested for each sample at the cross-head speed of 2.5 mm/min. Young’s modulus was calculated from the linear portion of strain/stress curves. Yield strength was determined by the 0.2% offset method.
3. Results 3.1. Microstructure of the HA/UHMWPE composite The ball milling and swelling process produced an unusual microstructure of the HA/UHMWPE composite. Fig. 3a shows that the cross section exhibits a
Fig. 3. SEM micrographs of ball-milled HA reinforced UHMWPE composite with swelling: (a) global view of cross-section showing a HA-rich zone (bright one) and a UHMWPE-rich zone (dark one); (b) microstructure of HA-rich zone (higher magnification of black rectangular in a); (c) HA penetration into polymer in the HA-rich zone and formed an interpenetration network (higher magnification of white rectangular in b); (d) microstructure of UHMWPE-rich zone (higher magnification of white rectangular in a).
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bicontinuous two-zone structure: the HA-rich zone (brighter zone) and the UHMWPE-rich zone (darker zone). Image analysis indicates that the HA content reaches 40 vol% in the HA-rich zone (Fig. 3b), in which the HA particles are dispersed uniformly into the UHMWPE matrix at the nano-scale, such level of dispersion can not be achieved if the HA size is not reduced. More importantly, the UHMWPE matrix in this region formed a network structure (Fig. 3c) attributed to the swelling treatment. In the UHMWPE-rich zone, however, the number density of HA particles is much lower (Fig. 3d). This is due to the large particle size ratio (UHMWPE/HA) and high viscosity of the UHMWPE melt. Thus, the HA particles could disperse homogenously throughout the composite only when the size of UHMWPE particles could be reduced to the same level as that of the HA particles, or the viscosity of UHMWPE could be decreased to allow the HA particles to penetrate inside the UHMWPE melt. We noted the size of UHMWPE particles could not be effectively reduced by ball milling because of their high toughness while swelling treatment could enhance the UHMWPE chain mobility and hence improve the processability of UHMWPE. The uniform dispersion of HA in the HA-rich zone is attributed to the effectiveness of ball milling. The HA particle size analysis shown in Fig. 4 reveals that ball milling reduced the HA particle size from 2 mm to under 100 nm. Note that as-received HA particles (Fig. 2) were agglomerations of a number of primary nanosized spherical particles due to the sintering process. Ball milling broke down the aggregates into individual nanosized particles which are the primary products of the wet synthesis. Without ball milling the micro-sized HA particles cannot penetrate into UHMWPE chains during swelling. TEM examinations provided information of the HA/ UHMWPE interface. Fig. 5 shows an image of the composite with swelling treatment and indicates that a number of HA particles do not show clear boundaries
Fig. 5. TEM micrograph of HA/UHMWPE composite with ball milling and swelling process.
Fig. 6. SEM micrograph of tensile fracture surface of swollen treated HA/UHMWPE.
Fig. 4. Size distribution of HA powder before and after ball milling.
between the matrix. Instead, the HA particles are surrounded by UHMWPE fibrils, which is attributed to the better chain mobility of the swollen UHMWPE. Without good adhesion between the matrix, the HA particles would exhibit sharp boundaries. The good adhesion was also revealed from the fracture surface shown in Fig. 6. The fracture path of a composite is commonly along the weak interface between the matrix and the reinforcement. Apparently, as shown in Fig. 6,
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Fig. 7. Comparison of mechanical properties: (a) Young’s modulus; and (b) Yield strength.
the fracture path of the swollen treated HA/UHMWPE composite mainly propagated in the matrix instead of along the interfaces. 3.2. Mechanical properties of the HA/UHMWPE composite The strengthening effects of HA reinforcement rely on the homogeneity of HA particle dispersion in UHMWPE. The improvement of HA dispersion by ball milling and swelling process should result in enhancing mechanical properties of HA/UHMWPE, which was confirmed by mechanical testing. Fig. 7 compared the Young’s modulus and yield strength of the composite, which was made by different combinations of ball milling and swelling treatment, with those of the control. Ball milling and swelling significantly enhanced the stiffness of the HA/UHMWPE composite, which achieved a 90% increase in Young’s modulus compared with pure UHMWPE as shown in Fig. 7a. Ballmilled HA particles improved the Young’s modulus of
the composite by 30%. Swelling further increased the modulus of the composite by 20–30%. Note that swelling also improved the modulus of pure UHMWPE by 15% because swelling reduced the entanglement density of UHMWPE thus improved its processability while the high viscosity of the unswollen one formed fusion defects. In comparison with the HA/HDPE composite, Young’s modulus of our HA/UHMWPE composite (1600 MPa) reached the same level as that of the saline treated HA/HDPE composite (20 vol% HA) prepared using twin screw compounding and hydroextrusion method [30–31], even though UHMWPE is not stiffer than HDPE [32]. HA particle reinforcement also nearly doubled the yield strength of pure UHMWPE as shown in Fig. 7b. Ball milling was more effective in improving the yield strength, and it increased the resistance to plastic deformation by 45%. Swelling, however, seems to have less influence on yield strength; it only increased plastic deformation resistance by a few percent. Probably swelling cannot increase the friction force among the polymer chains.
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steel balls. Certainly, the mixture would be even more uniform at nano scale unless the size of UHMWPE could be reduced to the same level of HA powder. Unfortunately, there’s no effective method to reduce the nascent UHMWPE particle size because of their high impact toughness. 4.2. Swelling effects on the interface structure and mechanical properties
Fig. 8. SEM micrograph of HA/UHMWPE mixture after compounding in ball mill (insert: global view, 300).
4. Discussion 4.1. Ball milling effects on the HA powder size and dispersion The experimental results indicate that ball milling enhances HA dispersion and the mechanical properties of HA/UHMWPE composites. Wet ball milling was found to effectively break the agglomeration of the nano-sized primary HA particles in the as-received micro-sized particles (Fig. 2) without changing its chemical nature (confirmed by XPS analysis). We found that ethanol was necessary in ball milling. It not only increased the number of collision events but also prevented the ball-milled powder from reaggregating. Eight hours of milling was sufficient to reduce the HA size to 50 nm. The results of mechanical testing seem to contradict the theories suggesting that the mechanical properties of a material are independent of the size of filler particles [8]. Obviously, size reduction increases the specific surface area and hence total surface energies which is favorable to interfacial interaction [9]. More importantly, nano-sized HA particles are much easier to diffuse into the UHMWPE matrix than the large HA particles, especially when swelling is applied. Wet ball milling of the HA and UHMWPE produced a uniform mixture. Ball milling is an effective mixing process by intensively stirring the powder and ball. Ethanol is useful as the dispersant to generate uniform mixture. SEM micrographs of HA/UHMWPE mixture (Fig. 8) after ball milling clearly show that the HA particles covered the UHMWPE surface uniformly, and a number of nano-sized HA particles were even found embedded in the micro-sized UHMWPE particles due to the large impact force between the HA powder and the
Swelling is a diffusion-controlled viscoelastic process driven by osmotic pressure in which small solvent molecules diffuse into the polymer matrix and induce disengagement between polymer chains. The process has been used to draw UHMWPE fiber by swell-drawing. Gao et al. [33] have systematically investigated the thermodynamics and kinetics of the swelling process of UHMWPE and found that swelling can effectively control the level of polymer chain entanglement density and improve drawability. The increase of chain mobility allows for better penetration of HA into the UHMWPE matrix. Paraffin oil, a saturated hydrocarbon, was used as swelling solvent because of its non-toxicity and small molecules with the same composition as UHMWPE. Swelling process increases the molecular spacing between UHMWPE chains. Therefore, chain mobility increases and chain relaxation becomes easier. The less entangled chains could envelop the HA particles. The open spaces resulting from extraction of paraffin oil were closed by compression at elevated temperatures. Since the powder size ratio of UHMWPE/HA was 1000, a completely uniform dispersion of HA in UHMWPE was not achievable. Thus, the wet ball milling and swelling processed composite exhibits a unique two-zone structure, a nano-sized HA particles reinforced UHMWPE with homogeneous distribution zone surrounding the continuous UHMWPE matrix zone as shown in Fig. 3a. Mechanical testing demonstrated this composite was stronger than that predicted by theoretical models, as will be elucidated below. Considering the HA-rich zone as a composite phase, its Young’s modulus can be estimated by the Guth–Smallwood model for particle reinforcement [34]. E c ¼ E m ð1 þ 2:5V f þ 14:1V 2f Þ: Here, V is the volume fraction, E is the Young’s modulus where subscripts c, m, f mean composite, matrix and filler, respectively. According to this model, the Young’s modulus of composite with 40 vol% HA particles should be 3620 MPa. The modulus of UHMWPE-rich zone can be estimated as that of pure UHMWPE (850 MPa). Knowing that the volume fraction of HA-rich phase is 20%, the Young’s modulus of the two-phase composite can be estimated by the rule of mixtures as 1400 MPa, which is lower than that of the experimental result (1630 MPa). The positive deviation
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from the Guth–Smallwood model might be attributed to dependency of the tensile modulus on the filler particle size. In fact, experiments have shown that the tensile modulus increases with decreasing particle size, and particle size distribution also has an effect on the tensile modulus since a mixture of different particle sizes increases packing efficiency and, therefore, the maximum packing volume fraction of fillers [35]. Increasing the zone with homogeneous HA dispersion particularly by reducing the initial UHMWPE powder size, may further enhance the mechanical properties of the composite; our future work will explore this possibility.
5. Conclusions In this work, a novel processing method of combining ball milling and swelling was developed for HA/ UHMWPE bio-composite. Ball milling in ethanol not only remarkably reduced HA particles to nano size but also effectively compounded HA/UHMWPE mixture uniformly. Swelling HA/UHMWPE composite in paraffin oil enhanced UHMWPE chain mobility and hence formed an interpenetrative network. The final HA/ UHMWPE composite exhibited a unique two-zone structure in which a homogenous HA/UHMWPE zone surrounded the UHMWPE-rich matrix. Ball milling and swelling treatment improved the mechanical properties of the HA/UHMWPE composites. Preliminary results of this novel processing indicate a promising direction for the future development of HA/UHMWPE processing for biomedical applications.
Acknowledgements This project was funded by the Research Grants Council of Hong Kong with an earmarked grant for research, Grant No. HKUST6244/00P. The research funds for High Impact Areas from The Hong Kong University of Science and Technology (HIA01/02.EG8 and HIA03/04.EG03) are also gratefully acknowledged. We thank Basell Ltd, USA for providing UHMWPE and the Engineering Research Center in Biomaterials at the Sichuan University in China for supplying HA particles. References [1] Bonfield W, Grynpas MD, Tully AE, Bowman J, Abram J. Hydroxyapatite reinforced polyethylene—a mechanically compatible implant material for bone replacement. Biomaterials 1981; 2(3):185–6. [2] Ramakrishna S, Mayer J, Wintermantel E, Leong KW. Biomedical applications of polymer-composite materials: a review. Compos Sci Technol 2001;61(9):1189–224.
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[3] Sousa RA, Reis RL, Cunha AM, Bevis MJ. Processing and properties of bone-analogue biodegradable and bioinert polymeric composites. Compos Sci Technol 2003;63(3–4): 389–402. [4] Oliveira AL, Leonor IB, Malafaya PB, Alves CM, Azevedo HS, Reis RL. Tailoring the bioactivity of natural origin inorganicpolymeric based systems. Bioceramics 15. Zurich-Uetikon: Trans Tech Publications Ltd; 2003. p. 111–42. [5] Wang M, Porter D, Bonfield W. Processing, characterization, and evaluation of hydroxyapatite reinforced polyethylene composites. Br Ceram Trans 1994;93(3):91–5. [6] Ladizesky NH, Ward IM, Bonfield W. Hydrostatic extrusion of polyethylene filled with hydroxyapatite. Polym Adv Technol 1997;8(8):496–504. [7] Wang M, Berry C, Braden M, Bonfield W. Young’s and shear moduli of ceramic particle filled polyethylene. J Mater Sci Mater Med 1998;9(11):621–4. [8] Wang M, Joseph R, Bonfield W. Hydroxyapatite-polyethylene composites for bone substitution: effects of ceramic particle size and morphology. Biomaterials 1998;19(24):2357–66. [9] Nazhat SN, Joseph R, Wang M, Smith R, Tanner KE, Bonfield W. Dynamic mechanical characterization of hydroxyapatite reinforced polyethylene: effect of particle size. J Mater Sci Mater Med 2000;11(10):621–8. [10] Wang M, Deb S, Bonfield W. Chemically coupled hydroxyapatite-polyethylene composites: processing and characterisation. Mater Lett 2000;44(2):119–24. [11] Wang M, Bonfield W. Chemically coupled hydroxyapatitepolyethylene composites: structure and properties. Biomaterials 2001;22(11):1311–20. [12] Sousa RA, Reis RL, Cunha AM, Bevis MJ. Coupling of HDPE/ hydroxyapatite composites by silane-based methodologies. J Mater Sci-Mater Med 2003;14(6):475–87. [13] Edidin AA, Kurtz SM. Influence of mechanical behavior on the wear of 4 clinically relevant polymeric biomaterials in a hip simulator. J Arthroplasty 2000;15(3):321–31. [14] Chan CM, Cheng CL, Yuen MMF. Electrical properties of polymer composites prepared by sintering a mixture of carbon black and ultra-high molecular weight polyethylene powder. Polym Eng Sci 1997;37(7):1127–36. [15] Thongruang W, Balik CM, Spontak RJ. Volume-exclusion effects in polyethylene blends filled with carbon black, graphite, or carbon fiber. J Polym Sci Part B-Polym Phys 2002;40(10): 1013–25. [16] Ruan SL, Gao P, Yang XG, Yu TX. Toughening high performance ultrahigh molecular weight polyethylene using multiwalled carbon nanotubes. Polymer 2003;44(19): 5643–54. [17] Wu QY, Wang X, Gao WP, Hu YL, Qi ZN. Unusual rheological behaviors of linear PE and PE/kaolin composite. J Appl Polym Sci 2001;80(12):2154–61. [18] Tong J, Ma YH, Jiang M. Effects of the wollastonite fiber modification on the sliding wear behavior of the UHMWPE composites. Wear 2003;255:734–41. [19] Xie XL, Tang CY, Chan KYY, Wu XC, Tsui CP, Cheung CY. Wear performance of ultrahigh molecular weight polyethylene/ quartz composites. Biomaterials 2003;24(11):1889–96. [20] Hashimoto M, Takadama H, Mizuno M, Yasutomi Y, Kokubo T. Titanium dioxide/ultra high molecular weight polyethylene composite for bone-repairing applications: preparation and biocompatibility. Bioceramics 15. Zurich-Uetikon: Trans Tech Publications Ltd; 2003. p. 415–8. [21] Roy S, Pal S. Characterization of silane coated hollow sphere alumina-reinforced ultra high molecular weight polyethylene composite as a possible bone substitute material. Bull Mater Sci 2002;25(7):609–12.
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[22] Suwanprateeb J. Binary and ternary particulated composites: UHMWPE/CaCO3/HDPE. J Appl Polym Sci 2000;75(12): 1503–13. [23] Anderson BC, Bloom PD, Baikerikar KG, Sheares VV, Mallapragada SK. At–Cu–Fe quasicrystal/ultra-high molecular weight polyethylene composites as biomaterials for acetabular cup prosthetics. Biomaterials 2002;23(8):1761–8. [24] Knets IV, Bunina LO, Filipenkov VV. Ultrahigh-molecular weight polyethylene and hydroxylapatite-based materials for replacement of bone tissue. Mech Compos Mater 1993;29(2):181–9. [25] Cunha AM, Reis RL, Ferreira FG, Granja PL. The influence of processing conditions on the mechanical behavior of UHMWPE/ HA and PMMA/HA composites. In: Kossowsky R, Kossowsky N, editors. June 19–July 2, Chania, Greece. Netherlands: Kluwer Academic Publishers; 1994. p. 163–76. [26] Reis RL, Granja PL, Cunha AM. Impact behavior of UHMWPE/HA composites for orthopedic prostheses. In: Ottenbrite RM, editor. Frontiers in biomedical polymer applications. Lancaster: PA Technomic Publishing; 1998. p. 251–63. [27] Gao P, Mackley MR. The structure and rheology of molten ultrahigh-molecular-mass polyethylene. Polymer 1994;35(24):5210–6. [28] Joseph R, McGregor WJ, Martyn MT, Tanner KE, Coates PD, Bonfield W. Effect of polymer matrix on the rheology of
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Biomaterials 26 (2005) 3471–3478 www.elsevier.com/locate/biomaterials
Processing of hydroxyapatite reinforced ultrahigh molecular weight polyethylene for biomedical applications Liming Fanga, Yang Lenga,, Ping Gaob a
Department of Mechanical Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China b Department of Chemical Engineering, The Hong Kong University of Science & Technology, Hong Kong, China Received 6 July 2004; accepted 8 September 2004
Abstract A new method for processing hydroxyapatite/ultrahigh molecular weight polyethylene (HA/UHMWPE) composite has been developed by combining wet ball milling and swelling. Sintered HA particles were ground in ethanol to 50 nm in diameter. The nano-sized HA particles were mechanically mixed with UHMWPE in the ball mill and then compression molded into solid slabs. The slabs were then swollen in a pharmaceutical grade paraffin oil to enhance the UHMWPE chain mobility and HA/UHMWPE interface adhesion before final hot press. The resultant composite exhibits a two-zone network structure formed by a homogeneous HA-rich phase and a UHMWPE-rich phase. This process resulted in a 90% increase in Young’s modulus and a 50% increase in the yield strength of HA/UHMWPE composite, comparing with those of unfilled UHMWPE. r 2004 Elsevier Ltd. All rights reserved. Keywords: Hydroxyapatite; UHMWPE; Composite; Swelling; Ball milling
1. Introduction Since Bonfield et al. [1] pioneered the use of hydroxyapatite (HA) to reinforce high density polyethylene (HDPE) for bone substitutes, there has been much interest in developing bio-analogue composites that have advantages over conventional artificial hardtissue replacement materials [2–3]. The mechanical and biological properties of bio-analogue composites can be tailored to meet specific clinical requirements [4]. Although HDPE has been widely used as the matrix for bio-composites [1,5–12], its wear resistance and impact strength are inferior to ultrahigh molecular weight polyethylene (UHMWPE) [13]. The use of UHMWPE as a composite matrix has been explored with various fillers, such as carbon (carbon black [14], carbon fiber [15], carbon nanotube [16], graphite [15]), ceramics (kaolin [17], wollastonite [18], Corresponding author. Tel.: +852 2358 7185; fax: +852 2358 1543.
E-mail address: [email protected] (Y. Leng). 0142-9612/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biomaterials.2004.09.022
quartz [19], TiO2 [20], A12O3 [21], CaCO3 [22], etc), or metals [23], etc. Generally, methods for processing UHMWPE composites include solid-state mixing and thermal forming [14–15,18,20–23], solution gelation and crystallization [16,19], and in situ polymerization [17]. Solid-state mixing and thermal forming process is easy to be performed but the filler/matrix interfacial strength is poor because only interaction between the filler and the matrix is mechanical interlocking, even though chemical surface treatment is employed [15,18,21–22]. Solution gelation and crystallization method is mainly utilized to produce an electrical conductive polymer blended with a very low fraction of carbon, but the mechanical properties are not the major concern. In situ polymerization of a UHMWPE composite requires strict processing conditions and has the problem of catalyst removal, which has only been tried for Kaolin/ UHMWPE composite [17]. Several researches on HA/UHMWPE composite have also been reported. Knets et al. [24] produced an annealed compact bone tissue reinforced UHMWPE
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composite, but few processing details were given. Cunha et al. [25] studied the influence of processing conditions on the mechanical behavior of the HA/UHMWPE composite using injection and compression molding, and found that injection moulding process assured better results than the compression molding due to the intensive mixing and shearing of the polymer. Reis et al. [26] investigated the impact behavior of the HA/UHMWPE composite. The polymer they used ¯ w ¼ 225; 000 g=mol; BP was Rigidex HM4560XP (M Chemicals Ltd., UK) [6], which actually is a type of HDPE. Use of UHMWPE as a composite matrix, however, has not achieved much success due to difficulties in processing. Its extremely high molecular weight makes it unprocessable by conventional thermoplastic processing techniques [27]. Furthermore, dispersion of HA particles in UHMWPE has been a serious challenge because of the latter’s extremely high viscosity [28], and UHMWPE is prone to fusion defects due to the long relaxation time of UHMWPE chains [29]. Therefore, it is necessary to develop new processing methods to fabricate a biocomposite with homogenous HA particles distribution in UHMWPE matrix and with interpenetration between HA particles and UHMWPE chains. In this study, we developed a novel method to process the HA/ UHMWPE composite by wet ball-mill compounding and swelling in a non-toxic solvent.
Fig. 1. SEM micrograph of as-received UHMWPE particle.
2. Materials and experimental methods The UHMWPE was the HiFax 1900 reactor powder (Basell Ltd, USA) with a molecular weight of 6 106 g/ mol and an average size of 200 mm (Fig. 1). The HA particles were the product of wet synthesis and sintering at 900 1C for 2 h (Fig. 2), provided by the Engineering Research Center for Biomaterials at Sichuan University, China. To investigate the effects of ball milling and swelling, four HA/UHMWPE samples were prepared using different combinations of processing techniques: (1) as-received HA reinforced UHMWPE unswollen treated; (2) ball-milled HA reinforced UHMWPE unswollen treated; (3) as-received HA reinforced UHMWPE swollen treated; and (4) ball-milled HA reinforced UHMWPE swollen treated. Unfilled UHMWPE specimens were also prepared in a similar manner for comparison. The ball milling was conducted in a rotary ball mill (PascallTM, Pascall Engineering Co. Ltd, Crawley, UK) with adjustable rotation speed up to 3000 rpm. The mill container volume was 1.5 L and the stainless steel ball diameter was 12.3 mm. The mill container and balls were cleaned by rinsing in an analytical grade ethanol (99.8%, Merck, Germany) before ball milling. In processing of sample 2 and 4, HA particles were ball
Fig. 2. SEM micrograph of as-received HA particles.
milled in ethanol for 8 h followed by drying in a vacuum oven at 80 1C overnight. And then the ball-milled HA powder (30 wt%) was compounded with UHMWPE reactor powder also by ball milling in ethanol for 4 h. Subsequently, the mixture was dried in a vacuum oven at 80 1C for 24 h. Afterwards, it was mechanically compacted into slabs of 38 mm in diameter and 1 mm in thickness at 135 1C. The swelling treatment of the as-compacted slab was carried out in pharmaceutical grade paraffin oil (Merck, Germany). The slab was immersed in paraffin oil at 135 1C for 10 h to reach a swelling ratio of 10. Then, the oil was removed by hot press (TMPs, Technical
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Machine Products, Ohio, USA) at 100 1C and further extracted by hexane (analytical grade, Merck, Germany) for 24 h. After extraction, the slab was air-dried overnight to allow the excess solvent to evaporate. Finally, the slab was hot pressed under 100 MPa at 180 1C to form HA/UHMWPE composite sheets. The size of HA particles was analyzed with Coulter LS230 (Beckman-Coulter, Florida, USA). The microstructure of the HA/UHMWPE composite was characterized using SEM (JEOL 6300, JEOL, Tokyo, Japan) and TEM (Philips CM 20, Philips Analytic, The Netherlands). The TEM samples were prepared by embedding a small piece of composite in epoxy and sectioning the embedded samples using ultracut-R ultramicrotome (Leica, Bensheim, Germany) with a diamond knife. The volume fraction of HA in the composite was measured from SEM micrographs by an image analyzer (Qwin Pro 2.2, Leica). Mechanical
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properties were evaluated by a universal testing machine (Instron 5500, Instron Co., Massachusetts, USA) with a load cell of 100 N. Tensile specimens were 50 mm long and 5 mm wide with a gauge length of 20 mm, which was approximately 0.2 mm thick. Six specimens were tested for each sample at the cross-head speed of 2.5 mm/min. Young’s modulus was calculated from the linear portion of strain/stress curves. Yield strength was determined by the 0.2% offset method.
3. Results 3.1. Microstructure of the HA/UHMWPE composite The ball milling and swelling process produced an unusual microstructure of the HA/UHMWPE composite. Fig. 3a shows that the cross section exhibits a
Fig. 3. SEM micrographs of ball-milled HA reinforced UHMWPE composite with swelling: (a) global view of cross-section showing a HA-rich zone (bright one) and a UHMWPE-rich zone (dark one); (b) microstructure of HA-rich zone (higher magnification of black rectangular in a); (c) HA penetration into polymer in the HA-rich zone and formed an interpenetration network (higher magnification of white rectangular in b); (d) microstructure of UHMWPE-rich zone (higher magnification of white rectangular in a).
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bicontinuous two-zone structure: the HA-rich zone (brighter zone) and the UHMWPE-rich zone (darker zone). Image analysis indicates that the HA content reaches 40 vol% in the HA-rich zone (Fig. 3b), in which the HA particles are dispersed uniformly into the UHMWPE matrix at the nano-scale, such level of dispersion can not be achieved if the HA size is not reduced. More importantly, the UHMWPE matrix in this region formed a network structure (Fig. 3c) attributed to the swelling treatment. In the UHMWPE-rich zone, however, the number density of HA particles is much lower (Fig. 3d). This is due to the large particle size ratio (UHMWPE/HA) and high viscosity of the UHMWPE melt. Thus, the HA particles could disperse homogenously throughout the composite only when the size of UHMWPE particles could be reduced to the same level as that of the HA particles, or the viscosity of UHMWPE could be decreased to allow the HA particles to penetrate inside the UHMWPE melt. We noted the size of UHMWPE particles could not be effectively reduced by ball milling because of their high toughness while swelling treatment could enhance the UHMWPE chain mobility and hence improve the processability of UHMWPE. The uniform dispersion of HA in the HA-rich zone is attributed to the effectiveness of ball milling. The HA particle size analysis shown in Fig. 4 reveals that ball milling reduced the HA particle size from 2 mm to under 100 nm. Note that as-received HA particles (Fig. 2) were agglomerations of a number of primary nanosized spherical particles due to the sintering process. Ball milling broke down the aggregates into individual nanosized particles which are the primary products of the wet synthesis. Without ball milling the micro-sized HA particles cannot penetrate into UHMWPE chains during swelling. TEM examinations provided information of the HA/ UHMWPE interface. Fig. 5 shows an image of the composite with swelling treatment and indicates that a number of HA particles do not show clear boundaries
Fig. 5. TEM micrograph of HA/UHMWPE composite with ball milling and swelling process.
Fig. 6. SEM micrograph of tensile fracture surface of swollen treated HA/UHMWPE.
Fig. 4. Size distribution of HA powder before and after ball milling.
between the matrix. Instead, the HA particles are surrounded by UHMWPE fibrils, which is attributed to the better chain mobility of the swollen UHMWPE. Without good adhesion between the matrix, the HA particles would exhibit sharp boundaries. The good adhesion was also revealed from the fracture surface shown in Fig. 6. The fracture path of a composite is commonly along the weak interface between the matrix and the reinforcement. Apparently, as shown in Fig. 6,
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Fig. 7. Comparison of mechanical properties: (a) Young’s modulus; and (b) Yield strength.
the fracture path of the swollen treated HA/UHMWPE composite mainly propagated in the matrix instead of along the interfaces. 3.2. Mechanical properties of the HA/UHMWPE composite The strengthening effects of HA reinforcement rely on the homogeneity of HA particle dispersion in UHMWPE. The improvement of HA dispersion by ball milling and swelling process should result in enhancing mechanical properties of HA/UHMWPE, which was confirmed by mechanical testing. Fig. 7 compared the Young’s modulus and yield strength of the composite, which was made by different combinations of ball milling and swelling treatment, with those of the control. Ball milling and swelling significantly enhanced the stiffness of the HA/UHMWPE composite, which achieved a 90% increase in Young’s modulus compared with pure UHMWPE as shown in Fig. 7a. Ballmilled HA particles improved the Young’s modulus of
the composite by 30%. Swelling further increased the modulus of the composite by 20–30%. Note that swelling also improved the modulus of pure UHMWPE by 15% because swelling reduced the entanglement density of UHMWPE thus improved its processability while the high viscosity of the unswollen one formed fusion defects. In comparison with the HA/HDPE composite, Young’s modulus of our HA/UHMWPE composite (1600 MPa) reached the same level as that of the saline treated HA/HDPE composite (20 vol% HA) prepared using twin screw compounding and hydroextrusion method [30–31], even though UHMWPE is not stiffer than HDPE [32]. HA particle reinforcement also nearly doubled the yield strength of pure UHMWPE as shown in Fig. 7b. Ball milling was more effective in improving the yield strength, and it increased the resistance to plastic deformation by 45%. Swelling, however, seems to have less influence on yield strength; it only increased plastic deformation resistance by a few percent. Probably swelling cannot increase the friction force among the polymer chains.
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steel balls. Certainly, the mixture would be even more uniform at nano scale unless the size of UHMWPE could be reduced to the same level of HA powder. Unfortunately, there’s no effective method to reduce the nascent UHMWPE particle size because of their high impact toughness. 4.2. Swelling effects on the interface structure and mechanical properties
Fig. 8. SEM micrograph of HA/UHMWPE mixture after compounding in ball mill (insert: global view, 300).
4. Discussion 4.1. Ball milling effects on the HA powder size and dispersion The experimental results indicate that ball milling enhances HA dispersion and the mechanical properties of HA/UHMWPE composites. Wet ball milling was found to effectively break the agglomeration of the nano-sized primary HA particles in the as-received micro-sized particles (Fig. 2) without changing its chemical nature (confirmed by XPS analysis). We found that ethanol was necessary in ball milling. It not only increased the number of collision events but also prevented the ball-milled powder from reaggregating. Eight hours of milling was sufficient to reduce the HA size to 50 nm. The results of mechanical testing seem to contradict the theories suggesting that the mechanical properties of a material are independent of the size of filler particles [8]. Obviously, size reduction increases the specific surface area and hence total surface energies which is favorable to interfacial interaction [9]. More importantly, nano-sized HA particles are much easier to diffuse into the UHMWPE matrix than the large HA particles, especially when swelling is applied. Wet ball milling of the HA and UHMWPE produced a uniform mixture. Ball milling is an effective mixing process by intensively stirring the powder and ball. Ethanol is useful as the dispersant to generate uniform mixture. SEM micrographs of HA/UHMWPE mixture (Fig. 8) after ball milling clearly show that the HA particles covered the UHMWPE surface uniformly, and a number of nano-sized HA particles were even found embedded in the micro-sized UHMWPE particles due to the large impact force between the HA powder and the
Swelling is a diffusion-controlled viscoelastic process driven by osmotic pressure in which small solvent molecules diffuse into the polymer matrix and induce disengagement between polymer chains. The process has been used to draw UHMWPE fiber by swell-drawing. Gao et al. [33] have systematically investigated the thermodynamics and kinetics of the swelling process of UHMWPE and found that swelling can effectively control the level of polymer chain entanglement density and improve drawability. The increase of chain mobility allows for better penetration of HA into the UHMWPE matrix. Paraffin oil, a saturated hydrocarbon, was used as swelling solvent because of its non-toxicity and small molecules with the same composition as UHMWPE. Swelling process increases the molecular spacing between UHMWPE chains. Therefore, chain mobility increases and chain relaxation becomes easier. The less entangled chains could envelop the HA particles. The open spaces resulting from extraction of paraffin oil were closed by compression at elevated temperatures. Since the powder size ratio of UHMWPE/HA was 1000, a completely uniform dispersion of HA in UHMWPE was not achievable. Thus, the wet ball milling and swelling processed composite exhibits a unique two-zone structure, a nano-sized HA particles reinforced UHMWPE with homogeneous distribution zone surrounding the continuous UHMWPE matrix zone as shown in Fig. 3a. Mechanical testing demonstrated this composite was stronger than that predicted by theoretical models, as will be elucidated below. Considering the HA-rich zone as a composite phase, its Young’s modulus can be estimated by the Guth–Smallwood model for particle reinforcement [34]. E c ¼ E m ð1 þ 2:5V f þ 14:1V 2f Þ: Here, V is the volume fraction, E is the Young’s modulus where subscripts c, m, f mean composite, matrix and filler, respectively. According to this model, the Young’s modulus of composite with 40 vol% HA particles should be 3620 MPa. The modulus of UHMWPE-rich zone can be estimated as that of pure UHMWPE (850 MPa). Knowing that the volume fraction of HA-rich phase is 20%, the Young’s modulus of the two-phase composite can be estimated by the rule of mixtures as 1400 MPa, which is lower than that of the experimental result (1630 MPa). The positive deviation
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from the Guth–Smallwood model might be attributed to dependency of the tensile modulus on the filler particle size. In fact, experiments have shown that the tensile modulus increases with decreasing particle size, and particle size distribution also has an effect on the tensile modulus since a mixture of different particle sizes increases packing efficiency and, therefore, the maximum packing volume fraction of fillers [35]. Increasing the zone with homogeneous HA dispersion particularly by reducing the initial UHMWPE powder size, may further enhance the mechanical properties of the composite; our future work will explore this possibility.
5. Conclusions In this work, a novel processing method of combining ball milling and swelling was developed for HA/ UHMWPE bio-composite. Ball milling in ethanol not only remarkably reduced HA particles to nano size but also effectively compounded HA/UHMWPE mixture uniformly. Swelling HA/UHMWPE composite in paraffin oil enhanced UHMWPE chain mobility and hence formed an interpenetrative network. The final HA/ UHMWPE composite exhibited a unique two-zone structure in which a homogenous HA/UHMWPE zone surrounded the UHMWPE-rich matrix. Ball milling and swelling treatment improved the mechanical properties of the HA/UHMWPE composites. Preliminary results of this novel processing indicate a promising direction for the future development of HA/UHMWPE processing for biomedical applications.
Acknowledgements This project was funded by the Research Grants Council of Hong Kong with an earmarked grant for research, Grant No. HKUST6244/00P. The research funds for High Impact Areas from The Hong Kong University of Science and Technology (HIA01/02.EG8 and HIA03/04.EG03) are also gratefully acknowledged. We thank Basell Ltd, USA for providing UHMWPE and the Engineering Research Center in Biomaterials at the Sichuan University in China for supplying HA particles. References [1] Bonfield W, Grynpas MD, Tully AE, Bowman J, Abram J. Hydroxyapatite reinforced polyethylene—a mechanically compatible implant material for bone replacement. Biomaterials 1981; 2(3):185–6. [2] Ramakrishna S, Mayer J, Wintermantel E, Leong KW. Biomedical applications of polymer-composite materials: a review. Compos Sci Technol 2001;61(9):1189–224.
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