Chemically coupled hydroxyapatite-polyethylene composites: processing and characterisation

Transcription

1 June 2000 Materials Letters Chemically coupled hydroxyapatite-polyethylene composites: processing and characterisation M. Wang ), S. Deb 1, W. Bonfield 2 Interdisciplinary Research Centre IRC in Biomedical Materials, Queen Mary and Westfield College, UniÕersity of London, London E1 4NS, UK Received 2 November 1999; accepted 20 December 1999 Abstract Hydroxyapatite Ž HA. -reinforced polyethylene was developed as a bone replacement material. In order to improve bonding between HA and polyethylene, and hence to increase mechanical properties of the composite, chemical treatments of HA and polyethylene were investigated and new composites manufactured. Two approaches were employed in this investigation: the use of silane-treated HA as the filler, and the application of polymer grafting for polyethylene. The silane coupling agent used was 3-trimethoxysiylpropylmethacrylate and the grafting monomer for polyethylene was acrylic acid. A processing route was established with and without the application of polymer grafting. New composites with different HA contents were produced and evaluated. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Hydroxyapatite; Silanation; High density polyethylene composites; Grafting; Processing; Characterisation 1. Introduction Hydroxyapatite Ž HA. -reinforced high density polyethylene Ž HDPE. composite is developed as a hard tissue replacement material wx 1, and has been ) Corresponding author. School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore Tel.: q ; fax: q address: mmwang@ntu.edu.sg Ž M. Wang.. 1 Present address: Department of Biomaterials, GKT Dental Institute, Guy s Tower, London Bridge, London SE1 9RT, UK. 2 Present address: Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK. successfully used in orbital surgery wx 2. Factors that affect thermal, physical and mechanical properties of polymer-matrix composites include matrix structure and molecular weight, reinforcement type and amount, and experimental conditions such as matrix modification and coupling agent treatment of the reinforcement. The adhesion of the reinforcement to the polymer matrix can have a significant influence on the characteristics and properties of the composite wx 3. The wetting of the surface of the reinforcement by the polymer is important; and generally, bonding of a particulate inorganic reinforcement Ž the filler. to the polymer depends on the hydrophilicity or polarity of the filler and the available polar groups of the polymer. A chemical interaction between the filler and the polymer will lead to much improved bond Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S X

2 120 M. Wang et al.rmaterials Letters ing, and hence mechanical properties. Polyethylene is a non-polar, hydrophobic polymer, and consequently, only mechanical interlocking exists between the HA particles and the polyethylene matrix in a conventionally processed HArHDPE composite wx 4. In order to achieve a good affinity between these two constituents of the composite, it is desirable for polyethylene to bear some polarity. Generally, a graft copolymerisation of a polar monomer, such as acrylic acid, onto polyolefins improves the adhesion between the filler and the polymer wx 5. The use of coupling agents for molecular bonding between a polymer matrix and inorganic fillers also enhances interfacial adhesion. In the present study, two approaches were used to improve the interfacial condition: the use of silane-treated HA as the filler and the application of polymer grafting for polyethylene. 2. Materials and methods The HDPE used in this investigation was Rigidex HM4560XP Ž BP Chemicals, UK., having a density of grcm 3 and a mean particle size of 1 mm. The HA powders used were of two grades: P81B HA and P88 HA, both commercially available from Plasma Biotal, UK Ž Table 1.. The silane coupling agent used was 3-trimethoxysiylpropylmethacrylate Ž A174, from Union Carbide, USA.. Analytical grade acrylic acid was used in the as-received condition without further purification. Two batches of new HArHDPE composites were manufactured. Feasibility studies were conducted on the first batch of material Ž composite A., with the second batch being produced in relatively large quantities for detailed structure and property investigations. The HA powder was silanated using A174 in a procedure which was described previously wx 6. Table 1 Properties of particulate hydroxyapatite for HArHDPE composites Hydroxyapatite Ca:P Ratio Mean particle Surface size Ž mm. Ž 2 area m rg. P81B HA P88 HA For polymer grafting, acrylic acid Ž5 wt.% relative to HDPE. and benzoyl peroxide were dissolved in acetone to form a homogenous solution to which was then added polyethylene granules. Acetone was evaporated by allowing the solution to stand overnight and the polyethylene granules were mixed with surface-treated HA subsequently. The mixture of HA and polyethylene was kneaded manually before compounding in order to break down any large agglomerates. The first batch of material consisted of silane-treated P81B HA and acrylic acid benzoyl peroxide polyethylene granules and contained 7 vol.% of HA. Compounding and grafting for the new HAr HDPE composites were performed on a Betol BTS 40L co-rotating twin screw extruder. This machine uses a pair of closely intermeshing screws to generate high shear forces to disperse hard particles in polymeric matrices. Enhanced micromixing is ensured when the material experiences a high level of shear as it flows through the screw channels. Such a facility is essential for producing highly filled polymer composites of good quality. The HArHDPE mixture was fed into the extruder manually and by carefully controlling the feeding speed, a stable motor current was maintained. A screw speed of 40 rpm and low temperature ranges Ž 1108C to 1408C. were used in the initial zones of the barrel to allow grafting. Temperatures of 1608C to 1908C were chosen for the subsequent zones. The relatively high screw speed was used so that materials could be conveyed quickly inside the barrel. The extruded composite wires were cooled in distilled water at room temperature, coiled and dried before pelletising. The compounded materials were subsequently powdered at the liquid nitrogen temperature in a centrifugal mill and compression moulded at ; 2008C into 140=140=1.75 mm 3 plates using a hydraulic press. After establishing the manufacturing route, relatively large quantities of new HArHDPE composites were produced, which contained 20 and 40 vol.% of the filler using silanated HA and non-grafted polyethylene or silanated HA and acrylic acid grafted polyethylene. Non-silanated and non-grafted HAr HDPE composite was also made for providing baseline data. Table 2 lists compositions of all HArHDPE composites produced in this investigation.

3 M. Wang et al.rmaterials Letters Table 2 Composition of HArHDPE composites Composite Filler Filler Silane Acrylic acid volume Ž %. coupling grafting A P81B HA 7 Yes Yes C P88 HA 20 Yes No D P88 HA 40 Yes No E P88 HA 20 Yes Yes F P88 HA 40 Yes Yes H P88 HA 20 No No K P88 HA 40 No No The percentage of grafting was determined using the powdered material before compression moulding. About 10.0 g of the composite powder was wrapped in a stainless steel net and refluxed in a solution of xylenermethanol Ž 2:1. for a period of 10 h. The solution was filtered and subsequently titrated against a 0.1 N methanolic KOH solution, which gave the weight Ž W. of the grafted acrylic acid Ž gaa. 1 and polyž acrylic acid.. In order to obtain the amount of polyž acrylic acid. formed, 5.0 g of the powdered composite was dissolved in xylene and filtered. The solution was then extracted with distilled water and the aqueous layer was titrated with methanolic KOH Ž 0.1 N. to obtain the weight Ž W. of the polyž 2 acrylic acid.. The amount of grafting was calculated as shown below: W1yW2 Percentage of grafting Ž %. s = 100 Ž 1. W 0 W1yW2 Efficiency of grafting Ž %. s = 100, Ž 2. W 1 where W0 was weight of the composite powder analysed. Infrared spectra of HArHDPE composites were obtained using a Nicolet 800 FTIR instrument in the diffuse reflectance mode. Melting endotherms and crystallisation exotherms of HArHDPE composites were recorded on a Perkin-Elmer DSC7 machine. The modular DSC7 system permitted direct measurement, characterisation and analysis of thermal properties of materials. The rate of cooling and heating was maintained at 208Crmin for all samples. These samples went through a first thermal cycle from ambient to 1908C, and then back to room temperature. Dwell at 1908C was 1 min. A second heating and cooling cycle was run in the same way. Crystallisation exotherms can be correlated to changes in crystallisation behaviour and the resulting polymer morphology. The analysis procedure used in this investigation was based on the work reported by Gupta et al. wx 7. Thermograms obtained in the second thermal cycles were used for analyses. The initial slope of the crystallisation peak Ž S. i depended on the rate of process occurring in the initial stages of crystallisation, i.e., nucleation. An increase in Si implied an increase in the rate of nucleation and vice versa. The width of the exotherm Ž Dw. at half height was dependent on the crystallite size distribution. The ratio of the area of the peak to the weight of the sample was proportional to the degree of crystallinity of a polymer. As HA is considered to be an inert filler, the area of the peak was normalised to the actual content of polyethylene. 3. Results and discussion Chemically coupled HArHDPE composites containing up to 40 vol.% of HA could be produced via the established manufacturing route. The percentage of grafting and the efficiency of grafting were determined according to the procedure described earlier. Fig. 1. Compounded and grafted HArHDPE composite.

4 122 M. Wang et al.rmaterials Letters Fig. 2. Infrared spectra of composites C and E. The efficiency of grafting was found to be 50% and the percent of grafting was 0.7%. Fig. 1 shows extrudates containing silanated HA and grafted polyethylene. Fig. 3. DSC thermograms of composites C, D, E and F the second heating cycle.

5 M. Wang et al.rmaterials Letters Fig. 4. DSC thermograms of composites C, D, E and F the second cooling cycle. Infrared spectra of chemically coupled HArHDPE composites showed that with an increase in the amount of silanated HA in the composite, the peak Ž y1 intensity in the carbonyl region cm. increased. This is an expected observation as the amount of HA in the composite increased more coupling agent was present in the composite. Fig. 2 displays infrared spectra of composites C and E. Composite E was grafted with acrylic acid whereas composite C had no acrylic acid grafted on the polyethylene. It was observed from these spectra that major differences lied in the carbonyl absorption in the cm y1 region. The carbonyl peak for the grafted composite appeared as a multiplet with distinct peaks at 1716 and 1728 cm y1, indicating the presence of different carbonyl groups arising from the acrylic acid and the silane coupling agent. Several peaks due to the C H bending frequencies were observed in the cm y1 region in the grafted composite. Infrared spectroscopy together with chemical analysis confirmed the grafting of acrylic acid onto polyethylene in the new HArHDPE composites. Fig. 3 exhibits endotherms of chemically coupled HArHDPE composites, suggesting that while the amount of HA did not affect the melting temperature Ž T. m of the composite, the presence of the coupling agent caused reductions in T m, which was further reduced when both grafting and silane coupling were present in the composite. This is possibly due to polymer chain extensions affecting the molecular spacing, flexibility and rotation. Chiang and Yang Table 3 Properties of chemically coupled HArHDPE composites as determined from DSC thermograms Composite Si Dw ArM a C D E F a The ratio of area of exotherm to sample mass with respect to HDPE.

6 124 M. Wang et al.rmaterials Letters reported similar trends for mica filled polypropylene composites wx 5. The Si of exotherms in the second thermal cycle Ž Fig. 4. was calculated. It was observed that, in general, an increase in the rate of nucleation Ž S. i resulted in the decrease of the crystal- lite size distribution Ž Dw.. With the introduction of grafting, there was a widening of the distribution of the crystallite sizes as compared to the material with the coupling agent only Ž Table 3.. The area under the crystallisation peak Ž A., when normalized to the weight of polyethylene Ž M. in the sample, indicates that with increasing amount of silanated HA in the composite the degree of crystallinity increased but that with the presence of grafting the degree of crystallinity decreased. 4. Conclusions Silanation of HA and acrylic acid grafting on polyethylene have been used for improving bonding between HA and polyethylene in HArHDPE composites. A processing route was established with and without the application of polymer grafting. The presence of silanation and grafting was confirmed by chemical and spectroscopic analyses. It is anticipated that with the chemical interaction between the filler and the matrix, mechanical properties of the new composites will be improved. Acknowledgements The support from the UK Engineering and Physical Sciences Research Council for the IRC programme is gratefully acknowledged. References wx 1 W. Bonfield, Composites for bone replacement, Journal of Biomedical Engineering 10 Ž wx 2 R.N. Downes, S. Vardy, K.E. Tanner, W. Bonfield, Hydroxyapatite-polyethylene composite in orbital surgery, in: Proceedings of the 4th International Symposium on Ceramics in Medicine, London, UK, 1991, p wx 3 S.F. Xavier, J.M. Schultz, K. Friedrich, Fracture propagation in particulate-filled polypropylene composites: Part 3. Influence of mica surface treatment, Journal of Materials Science 25 Ž wx 4 M. Wang, D. Porter, W. Bonfield, Processing, characterisation and evaluation of hydroxyapatite reinforced polyethylene composites, British Ceramic Transactions 93 Ž wx 5 W.Y. Chiang, W.D. Yang, Polypropylene composites: I. Studies of the effect of grafting of acrylic acid and silane coupling agent on the performance of polypropylene mica composites, Journal of Applied Polymer Science 35 Ž wx 6 S.N. Khorasani, S. Deb, J.C. Behiri, M. Braden, W. Bonfield, Modified hydroxyapatite reinforced PEMA bone cement, in: Proceedings of the 5th International Symposium on Ceramics in Medicine, Kyoto, Japan, 1992, p wx 7 A.K. Gupta, S.K. Rana, B.L. Depura, Crystallisation behaviour of high density polyethylenerlinear low density polyethylene blend, Journal of Applied Polymer Science 44 Ž