228 PIRHONEN ET AL. MATERIALS AND METHODS elemental analyses were performed, one on manufactured bioactive glass fibers and the other on the remnants of the ing of the Glas glass in the platinum crucible The bioactive glass 13-93 contains 6 wt Na,o, 12 wt %o Tensile Test. Tensile tests were performed on the newly K2O, 5 wt MgO, 20 wt Cao, 4 wt P, Os, and 53 wt drawn bioactive glass fibers, normally wit n davs after the 96 SiO 2. The raw materials used, analytical grade Na- CO3, manufacturing. The tensile test results reported in this study weighed and mixed in a plastic container. The mixed raw are results from various manufacturing batches. The ASTM materials were melted in a platinum crucible for 3 h at D3379-75 instructions were followed in the tensile testing, in hieve a homogeneous glass the main. A gauge length of 50 mm and a testing speed of 1 was crushed into mm/min were used. The Instron materials testing machine m' pieces and reheated for 3 h at(Series IX Automated Materials Testing System 1.38)with either a 10N or a 500N load cell was used, depending on the Processing of Glass Fibers thickness of the fiber. The diameter of the fiber was measured from both free ends of the sample with a micrometer with an The glass fiber manufacturing machinery was built to perform accuracy of 0.001 mm. The average of the two measured melt spinning tests. The apparatus contained a furnace with values was used in calculations of the strength. The diameter an opening at the bottom, as well as a fiber spinning unit, of the fibers varied from approximately 25 to 180 um. Prior which was placed -1. 5 m below the furnace to enable the to the testing, care was taken not to touch the fibers to avoid fibers to fully cool down before spinning into a roll. The any contamination of fiber surfaces. Since bioactive glass speed of the spinning roll was controlled by adjusting the fibers are brittle, they do not have a well-defined tensile rolling speed of the motor and by changing the roll diameter. strength. The stress at which they fail depends mostly on the To perform the melt spinning of the fibers, the formed glass presence of flaws, which may occur randomly along the block was heated in a platinum crucible with seven orifices at length of the fiber. The Weibull distribution is a statistical e bottom. After a while, the glass melt started to drain model used to describe the scatter in strength data. slowly, forming continuous fibers from each of the nozzles. strength values are ranked in descending order; the highest The drained glass drops/fibers were attached to a spinning roll strength value having the rank j=l, the next j=2, and so on creating continuous fibers. The achieved fiber diameter could until the smallest value j=N. a likelihood to survive, be adjusted from approximately 20 up to 300 um. The Sj=j/(N+1), is then assigned to each value of strength. A optimum fibrillation parameters were previously studied in minimum of 30 values of fracture strength are required for the project by variation of the melt temperature and the statistical validity. A Weibull's diagram is obtained by plot diameter of the orifices. In this study, a furnace set temper- ting InIn(1/S)) against In(strength). The gradient of the line of ature of 960C and an orifice nozzle diameter of -3.7 mm regression on In In(1/s)) upon In(strength) is the Weibul were found to be optimal for the melt spinning of glass modulus m, which enables the probability of a material to 3-93 survive under a given stress to be estimated. The Weibull modulus of glass 13-93 fibers were evaluated as outlined SBF Study earlier To study fiexural strength retention, changes of mass, and Strength Retention. The flexural strength was measured surface reactions in vitro, fiber samples were immersed in a using a three-point bending test fixture on fibers with diam- SBF by Kokubo. Fiber samples were kept in closed plastic containers stored in thermo closet at +37C. The sample eters from 229 to 338 um. A span length of 3. 8 mm and the surface area to SBF volume(SA/V) ratio of 0.1 cm was crosshead speed of 0.5 mm/min were used in this testing Prior to the testing, care was taken to avoid any contamina- used for all the test samples. The pH of the SBF solution was tion of the fiber surfaces. After immersion in SBF, the sam- sets of analysis were performed for samples with in vitro ples were flushed with distilled water and kept moist until the conditioning, namely (1)flexural strength retention of fiber testing was performed. The Weibull modulus of bioactive rods,(2)change in mass, and (3)scanning electron micro- glass 13-93 fibers was evaluated, as outlined earlier for flexural strength series where the number of samples was ic(SEM)analysis and compositional analysis of the above 30. Also the average and median strength and modulus glass surface after immersion in SBF. values were calculated for fiber series immersed in Sbf for Tests and Analysis Performed various time periods Elemental Analyses. Elemental analyses were performed Changes in Mass. Three series of fiber samples were find out whether there was a change in the glass compo- prepared, namely fibers with a nominal fiber diameter of 38, sition during the fiber manufacturing process. The elemental 100, and 210 um. The surface area/volume(SA/) ratio of analyses were performed by Rautaruukki Steel(Raahe, Fin- 0. 1 cm was used for all tested samples. The surface area of land) with X-ray spectroscopy(Philips PW 2404 RGT). Two samples was kept constant with all the samples, and so theMATERIALS AND METHODS Manufacturing of the Glass The bioactive glass 13–93 contains 6 wt % Na2O, 12 wt % K2O, 5 wt % MgO, 20 wt % CaO, 4 wt % P2O5, and 53 wt % SiO2. The raw materials used, analytical grade Na2CO3, K2CO3, MgO, CaCO3, CaHPO4
2H2O as well as SiO2, were weighed and mixed in a plastic container. The mixed raw materials were melted in a platinum crucible for 3 h at 1360°C. To achieve a homogeneous glass, the formed glass was crushed into
1 cm3 pieces and reheated for 3 h at 1360°C. Processing of Glass Fibers The glass fiber manufacturing machinery was built to perform melt spinning tests. The apparatus contained a furnace with an opening at the bottom, as well as a fiber spinning unit, which was placed
1.5 m below the furnace to enable the fibers to fully cool down before spinning into a roll. The speed of the spinning roll was controlled by adjusting the rolling speed of the motor and by changing the roll diameter. To perform the melt spinning of the fibers, the formed glass block was heated in a platinum crucible with seven orifices at the bottom. After a while, the glass melt started to drain slowly, forming continuous fibers from each of the nozzles. The drained glass drops/fibers were attached to a spinning roll creating continuous fibers. The achieved fiber diameter could be adjusted from approximately 20 up to 300
m. The optimum fibrillation parameters were previously studied in the project by variation of the melt temperature and the diameter of the orifices. In this study, a furnace set temper￾ature of 960°C and an orifice nozzle diameter of
3.7 mm were found to be optimal for the melt spinning of glass 13–93. SBF Study To study flexural strength retention, changes of mass, and surface reactions in vitro, fiber samples were immersed in a SBF by Kokubo.13 Fiber samples were kept in closed plastic containers stored in thermo closet at 37°C. The sample surface area to SBF volume (SA/V) ratio of 0.1 cm1 was used for all the test samples. The pH of the SBF solution was monitored and solution changed once in every 2 weeks. Three sets of analysis were performed for samples with in vitro conditioning, namely (1) flexural strength retention of fiber rods, (2) change in mass, and (3) scanning electron micro￾scopic (SEM) analysis and compositional analysis of the glass surface after immersion in SBF. Tests and Analysis Performed Elemental Analyses. Elemental analyses were performed to find out whether there was a change in the glass compo￾sition during the fiber manufacturing process. The elemental analyses were performed by Rautaruukki Steel (Raahe, Fin￾land) with X-ray spectroscopy (Philips PW 2404 RGT). Two elemental analyses were performed, one on manufactured bioactive glass fibers and the other on the remnants of the glass in the platinum crucible. Tensile Test. Tensile tests were performed on the newly drawn bioactive glass fibers, normally within days after the manufacturing. The tensile test results reported in this study are results from various manufacturing batches. The ASTM D3379 –75 instructions were followed in the tensile testing, in the main. A gauge length of 50 mm and a testing speed of 1 mm/min were used. The Instron materials testing machine (Series IX Automated Materials Testing System 1.38) with either a 10N or a 500N load cell was used, depending on the thickness of the fiber. The diameter of the fiber was measured from both free ends of the sample with a micrometer with an accuracy of 0.001 mm. The average of the two measured values was used in calculations of the strength. The diameter of the fibers varied from approximately 25 to 180
m. Prior to the testing, care was taken not to touch the fibers to avoid any contamination of fiber surfaces. Since bioactive glass fibers are brittle, they do not have a well-defined tensile strength. The stress at which they fail depends mostly on the presence of flaws, which may occur randomly along the length of the fiber. The Weibull distribution is a statistical model used to describe the scatter in strength data.14 N strength values are ranked in descending order; the highest strength value having the rank j1, the next j2, and so on until the smallest value j  N. A likelihood to survive, Sjj/(N1), is then assigned to each value of strength. A minimum of 30 values of fracture strength are required for statistical validity. A Weibull’s diagram is obtained by plot￾ting ln ln(1/Sj) against ln(strength). The gradient of the line of regression on ln ln(1/Sj) upon ln(strength) is the Weibull modulus m, which enables the probability of a material to survive under a given stress to be estimated. The Weibull modulus of glass 13–93 fibers were evaluated as outlined earlier. Strength Retention. The flexural strength was measured using a three-point bending test fixture on fibers with diam￾eters from 229 to 338
m. A span length of 3.8 mm and the crosshead speed of 0.5 mm/min were used in this testing. Prior to the testing, care was taken to avoid any contamina￾tion of the fiber surfaces. After immersion in SBF, the sam￾ples were flushed with distilled water and kept moist until the testing was performed. The Weibull modulus of bioactive glass 13–93 fibers was evaluated, as outlined earlier for flexural strength series where the number of samples was above 30. Also the average and median strength and modulus values were calculated for fiber series immersed in SBF for various time periods. Changes in Mass. Three series of fiber samples were prepared, namely fibers with a nominal fiber diameter of 38, 100, and 210
m. The surface area/volume (SA/V) ratio of 0.1 cm1 was used for all tested samples. The surface area of samples was kept constant with all the samples, and so the 228 PIRHONEN ET AL.