Structure, viscosity and fibre drawing properties of phosphate-based glasses: effect of boron and iron oxide addition

Resorbable phosphate-based glasses have been applied as fibrous reinforcement for resorbable polymers for fracture fixation. The mechanical properties of these composites largely depend on the mechanical properties of the fibres. In this current study, four phosphate-based glass compositions were produced by replacing Na2O with B2O3 and/or Fe2O3 in the glass system P2O5–CaO–Na2O–MgO, and the P2O5 content was fixed at 45 mol%. The thermal stability of the glasses containing both B2O3 and Fe2O3 and/or FeO (P45B5Fe5 and P45B5Fe3) was significantly higher than that of the only B2O3 (P45B5)- or Fe2O3 (P45Fe3 and P45Fe5)-containing glasses. The viscosity was found to shift to higher temperature with increasing B2O3 and Fe2O3 and/or FeO content. The fragility parameters, m and F1/2, estimated from the viscosity curve, decrease with B2O3 addition. The improved physical properties of the glasses investigated with B2O3 and Fe2O3 and/or FeO addition were attributed to the replacement of P–O–P bonds with P–O–B and P–O–Fe bonds. The presence of P–O–B and P–O–Fe bonds in the glass structure was confirmed by the FTIR analysis. It was possible to draw continuous fibres up to 3 h from the B2O3- or Fe2O3- and/or FeO-containing glasses, whereas it was difficult to pull fibre from only Fe2O3-containing glasses and the fibre pulling process was not continuous. Therefore, addition of B2O3 to the glass system enabled successful drawing of continuous fibres from glasses with phosphate (P2O5) contents of 45 mol%. It was also observed that addition of only Fe2O3 and/or FeO did not have a significant effect on the fibre mechanical properties, whilst the mechanical properties of the fibres increased with increasing B2O3.

INTRODUCTION Phosphate based glasses without silica and with high CaO/P2O5 molar ratio have a great potential to be used for biomedical applications as their chemical composition is close to that of natural bone. However, a very high temperature is required to prepare these glasses and often have the tendency to crystallise [1]. Therefore, different modifier oxides have been added to PBGs to improve the thermal stability and durability of the glasses to suit the end application. Phosphate based glasses (PBGs) have the property of being completely soluble in aqueous medium and their degradation rate can easily be altered via addition of different modifier oxides. These unique physical and chemical properties of PBGs have attracted huge interest in their use within the field of biomaterials and tissue engineering [2][3][4][5].
In recent years, iron phosphate glasses have received increasing attention because of their high thermal stability, chemical durability, high compositional flexibility and low melting points. These characteristics of iron phosphate glasses have prompted substantial research in the past decade aimed at utilising them as hosts for the immobilisation of toxic and nuclear wastes [6][7][8]. More recently, there has been a growing interest in using the iron phosphate glasses for different biomedical applications [9,10]. However, a number of studies have shown that the iron phosphate glasses tend to crystallise at relatively low temperatures, which is a common problem associated with the drawing of PBG fibres [8], as they would tend to crystallise at the working process temperature [11], particularly with Q 0 and Q 1 dominated structures. Mössabauer spectroscopy revealed that iron can be present as both Fe 2+ and Fe 3+ in iron phosphate glasses and the concentration of Fe 2+ decreased with increasing Fe2O3 content [8].
B2O3 is known to improve the thermal stability of PBGs by suppressing their tendency to crystallise by altering the dimensionality of the phosphate network via the formation of long chain Q 2 species rather than smaller Q 0 or Q 1 units [1,12,13]. Moreover, it has been reported that the addition of 5 and 10 mol% of B2O3 to the phosphate glasses increased the thermal stability by reducing the tendency to crystallise [14].With this in mind, it was hypothesised that the addition of B2O3 content in the iron phosphate glass formulation could improve the thermal properties of the glasses by forming long chain structures.
The addition of B2O3 to PBGs has been reported to significantly improve the thermal stability and durability of the glass systems [14]. It has also been reported that the addition of B2O3 made the fibre production a continuous process with greater ease [15,16]. Therefore, the aim 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 of the current study was to investigate the effect of B2O3 addition on the thermal, structure, viscosity and fibre drawing properties of PBG glasses in the system P2O5-CaO-MgO-Na2O-Fe2O3 with phosphate contents fixed at 45 mol%. The main reason for attempting to achieve this was that previous studies from the group had shown a much more favourable cytocompatible response for PBG formulations containing Fe2O3, however proved very difficult to fiberise. The results from this study showed that continuous fibres from PBG formulations with P2O5 contents of 45mol% could be fiberised continuously without breaking with addition of B2O3. In addition, the effect of B2O3 and/or Fe2O3 addition on the mechanical properties of the as drawn fibres was also evaluated.

Thermal Analysis
Glass pieces of the various compositions were ground to fine powder using a pestle and mortar. The glass transition temperature (Tg) of the glasses was determined using differential scanning calorimeter (DSC, TA Instruments Q10, UK). A sample of each glass composition was heated from room temperature to 520 o C at a rate of 20 o C min --1 in flowing argon gas.
The Tg was extrapolated from the onset of change in the endothermic reaction of the heat flow [17].
To determine the onset of crystallisation a different DSC instrument (TA Instruments SDT Q600, UK) was used. Samples were heated from room temperature to a value of Tg+20 o C at a rate of 20 o C min -1 , held there isothermally for 15 min and then cooled down at a rate of 10 o C min -1 to 40 o C before ramping up again to 1100 o C a rate of 20 o C min -1 under flowing argon gas. The samples were subjected to the programmed heating cycle to introduce a known thermal history. A blank run was carried out to determine the baseline which was then subtracted from the traces obtained. The Tg was determined from the second ramping cycle in the same process discussed above. The first deviation of the DSC curve from the baseline above Tg before crystallisation peak was taken as the onset of crystallisation temperature. The Thermal stability of the glasses was measured in terms of the processing window by taking the temperature interval between Tg and the onset of crystallisation temperature (Tc,ons) as shown in Equation 1 below.

Fourier transform infrared (FTIR) spectroscopic analysis
Infrared spectroscopy was performed on a Bruker Tensor-27 spectrometer (Germany). All spectra were analysed using Opus TM software version 5.5. The glass samples were crushed and ground into fine powder using a mineral mortar and pestle. The samples were scanned in absorbance mode in the region of 4000 to 550 cm -1 (wave numbers) using standard Pike attenuated total reflectance (ATR) cell (Pike technology, UK).

Viscosity/temperature measurements
A parallel plate method was used for viscosity (η)/temperature measurements in the range of 8 Pa s < log (η) < 5.5 Pa s using a thermo-mechanical analyser (Perkin Elmer where, F is the applied force, h is the sample height at time t and V is the sample volume. At high temperatures a rotational viscometer was used (Brookfield DV-ΙΙΙ UTRA, USA). The viscosity was measured by measuring the force required to rotate a spindle in the molten glass. The viscosity was determined by measuring the shear stress and the shear rate exerted by the viscous fluid on a rotating cylindrical platinum spindle according to: where, η is the viscosity in poise,  is the rate of shear in sec -1 and  is the shear stress in dynes/cm 2 . Calibration was undertaken by use of the borosilicate glass standard reference material 717a which has been described by Parsons et al. [19].
In order to derive a viscosity curve for each of the nine glasses, the data points from both the parallel plate method and rotational method were combined and fitted to the Fulcher equation by a least squares calculation [20]. The equation is: where, T is the temperature in o C, η is the viscosity in Pa s and A, B and To are constants. The values of the constants/parameters calculated for each of the samples are given in Table 4.
Fragility is a qualitative concept which addresses the deviations of liquid relaxation times from Arrhenius behaviour [21]. In order to compare the viscosity/temperature behaviour of  An alternative fragility index (F1/2) has also been introduced as [23]: where T1/2 is the temperature when the viscosity is halfway (log η=3.5) (on logarithmic scale) between log η=12 (characteristics of the glass transition temperature for non-fragile liquids) and log η=-5 ( which is the roughly common high-temperature limiting value) [23]. The values of m and F1/2 calculated for each of the samples are given in Table 5.

Fibre drawing process
Continuous fibres approximately ~20 m diameter were produced via a melt-draw spinning process using a dedicated in-house facility. Figure 1 shows the SEM images of the as drawn fibres. The pulling temperature was adjusted to around 1150 o C. The molten glass was pushed though the bushing by hydrostatic pressure and was collected on a rotating drum.

Single fibre filament test
Single fibre filament tests (SFTT) were conducted in accordance with ISO 11566 [24].
Twenty fibres were mounted individually onto plastic tabs for each sample, with a 25mm gauge length testing setup. The ends of each fibre were bonded to the plastic tab with an acrylic adhesive (Dymax 3099 -Dymax, Europe) and the adhesive was cured using UV light.
In order to determine the individual diameter of each fibre prior to testing, the fibre specimens were measured by using a laser scan micrometer, LSM 6200 (Mitutoyo, Japan). 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 The laser scan micrometer was calibrated with glass fibre of known diameters (determined by SEM) and the error on diameter measurements is considered to be ±0.3 m. The SFTT was performed using a LEX810 Tensile Tester (UK) at room temperature with a load capacity of 0.2 N and a speed of 0.017 mm s -1 . The student's t-test was used to study the effect of composition on the tensile fracture stress and modulus values of the fibres. Significance was detected at a 0.05 level and all statistical analysis was carried out using GraphPad Prism for Windows (GraphPad, Software Inc, USA).
The Weibull distribution is a well-known and accepted method to describe the strength of fibres [25]. Weibull modulus and normalising stress are found statistically as the shape and scale factors. The normalising stress σo can be regarded as the most probable stress at which a fibre of length Lo will fail. PBG fibres are essentially brittle and Weibull distribution is an accepted statistical tool used to characterize the failure mode of brittle fibres. In this study, Weibull parameters were obtained from the tensile fracture stress data calculated using Minitab ® 15 (version 3.2.1). Table 3 shows the effect of B2O3 (5 mol%) and Fe2O3 ( The values for the processing window (Tc,ons -Tg), which is also an indication of the thermal stability for glasses, are presented in Figure 2. The values for the processing window were seen to increase as B2O3 was added to the P45Fe3 and P45Fe5 glass systems. With 5 mol% B2O3 addition, the processing window increased from 72 o C to 77 o C and 71 o C to 80 o C for P45Fe3 and P45Fe5 glass formulations, respectively .   1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63 64 65

Fourier transform infrared (FTIR) spectroscopy
The structural properties of the glasses as a function of B2O3 content were investigated using infrared spectroscopy. The IR spectra of glasses can be seen in Figure 3. The four bands in the IR spectra of only Fe2O3 containing glasses (P45Fe3 and P45Fe5) were observed at 730 cm -1 , 908 cm -1 , 1097 cm -1 and 1245 cm -1 . It was observed that the intensity of the band detected at 730 cm -1 of the P45Fe3 and P45Fe5 glasses decreased and shifted to the higher wave number (742 cm -1 ) as B2O3 was added to the glass systems. A similar variation was observed for the band at 908 cm -1 . The band shifted to the higher wave number (920 cm -1 ) and the intensity of the band also decreased as 5 mol% B2O3 was added to P45Fe3 and P45Fe5 glass systems. The intensity of the bands at 1097 cm -1 and 1245 cm -1 were also seen to decrease with B2O3 addition. However, this time the bands shifted to a lower wave number.
Viscosity/ temperature analysis Figure 4 shows the measured Log η as a function of temperature. At high temperature, the viscosities of the glass forming liquids containing boron were greater than those with no boron. However, there appeared to be a convergence between P45Fe3 and P45Fe5; P45B5Fe3 and P45B5Fe5 at lower temperature.
Whereas, no significant difference (P >0.05) in tensile strength was observed as Fe2O3 content alone was increased from 3 to 5 mol%. The tensile strengths of P45Fe3 and P45Fe5 were 511 ± 121 and 526 ± 110 MPa, which increased to 997±184 and 1003±193 MPa for P45B5Fe3 and P45B5Fe5 glass formulations, respectively.

Thermal Analysis
The thermal stability of the glasses in terms of processing window was found to increase with increasing B2O3 content ( Figure 2). The processing window increased by 7% and 11% as 5 mol% B2O3 was added to the P45Fe3 and P45Fe5 glass systems, respectively. The reduction in the number of non-bridging oxygens was also suggested to be responsible for raising the temperature of the onset of crystallisation [26], which will eventually increase the processing window of the glasses. Harada et al. suggested that the addition of B2O3 suppressed the formation of orthophosphate Q 0 units, which promoted crystallisation [13]. They also suggested that addition of B2O3 could supress surface crystallisation due to the formation of highly cross-linked structure based on metaphosphate Q 2 tetrahedra. Therefore, it is suggested that addition of B2O3 to the phosphate glass network could alter the structure dimensionality of the phosphate network via the formation of chain-like Q 2 species rather   1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 than Q 0 or Q 1 units, which in turn improved the processing window of the glasses as seen in Figure 2.

Fourier transforms infrared (FTIR) spectroscopy
The IR spectra (see Figure 3) showed the structural changes of the iron phosphate glasses due to B2O3 addition. When B2O3 is added to iron phosphate glasses, both P-O-B and P-O-Fe bonds will exist in the glass [27]. The bands observed at 729 cm -1 and 908 cm -1 in the IR spectra of the base iron phosphate glasses were assigned to symmetric and asymmetric stretching of P-O-P bridging bonds, respectively [28]. The band at 1245 cm -1 is assigned to the asymmetric vibrations of the non-bridging oxygen atoms in the phosphate chains.
With the addition of B2O3 the band at 1245 cm -1 became broader and the intensity reduced; this decrease in intensity reflects a reduction in the number of non-bridging P-O bonds and was indication of a progressive increase in the connectivity [29]. In borate glasses, the region around 850-1200 cm -1 was attributed to the B-O stretching of BO4 units [30]. Therefore, it was likely that this connectivity was due to the replacement of P-O-P bonds with P-O-B links. The broadening of the bands in the region of 908 cm -1 also suggested formation of P-O-B bands as B2O3 was added. Both bands for asymmetric and symmetric stretching of P-O-P bridging oxygens shift to higher frequency as B2O3 was substituted for Na2O. The absorption band near 1098 cm -1 have been assigned to P-Ogroups (chain terminators) [31]. The P-Oabsorption bands near 1098 cm -1 shifts to lower frequency as B2O3 replaces Na2O. Similar shift for P-Oabsorption bands to lower frequency was also observed by Bartholomew et al. for silver metaphosphate glasses [31]. They attributed such shift to the existence of covalent bonds between silver ions and the non-bridging oxygen.

Viscosity/ temperature
The viscosity/temperature plot was found to shift to higher temperature as B2O3 was added to the P45Fe3 and P45Fe5 glass forming liquids (see Figure 4). In general, the viscosity η is affected by the bonding energy between the cations and oxygens in the glass structure [32].
Therefore, the shift of viscosity/temperature plot to higher temperature region was expected as the structure of the glass forming liquids became strongly bonded when Na2O was replaced by B2O3. Toyoda et al. studied the viscosity behaviour of several binary glass systems (50RO-50P2O5 ;R=Sr, Ca, Zn and Mg) and reported that the viscosity/ temperature curve shifted to higher temperature in the order of Ba<Sr<Ca<Mg which was similar to the order of 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 field strength of the respective ions [33]. They suggested that the structural rigidity of the glass structure increased with increasing cationic field strength which in turns increased the temperature/viscosity curve to higher temperature. A study of alkaline earth zinc phosphate glasses in the series of 20MO-30ZnO-50P2O5 (M=Br, Sr, Ca, Mg) by Striepe and Deubener also confirmed the effect of increasing field strength on the shift of viscosity / temperature curve to higher temperature [34]. They found that the fragility index m decreased with increasing field strength of the cations. The activation energy of viscous flow, which is an indication of energy required to sever sufficient bonds within the glass network to initiate flow [35], is strongly affected by the cross-linking [36] and chain length [37]. Sharifah et al.
studied the effect of phosphate chain length on the activation energy of the viscous flow and reported that glass formulations with shorter chain lengths showed lower activation energy for viscous flow and vice versa [37]. Gray and Klein reported that the viscosity of phosphate glasses increased with increasing crosslinking [36]. Therefore, the shift of the viscosity/temperature plot to higher temperature with addition of B2O3 could be attributed to the fact that addition of boron to the phosphate glass network increased the cross-linking density and chain lengths by becoming or forming part of the glass network as also evidenced by the higher Tg and enhanced processing window.
The glass forming liquids can be characterised as strong or fragile based on the fragility index in which strong liquids exhibits low m values near Tg in the fragility plot and vice versa for the fragile liquids. The fragility is dependent on glass network polymerisation, which is highly altered by the addition of different modifying oxides [33]. The kinetic fragility parameter, m and F1/2 estimated from the viscosity curve were found to decrease with the addition of B2O3. The decreasing value of fragility is an indication that the glass network is transforming from a fragile to strong network with the addition of B2O3. Richardson et al.
studied the viscosity properties of sodium borophosphate glasses in the system of (1- x)NaPO3-xNa2B4O7 and found that the kinetic fragility parameter decreased with increasing sodium borate content [35]. They suggested that the decreasing fragility with increasing sodium borate content was due to the progressive depolymerisation of the phosphate network by the four coordinated boron atoms present in the glass network. The constant fragility may indicate that the coordination environment as well as the M-O-P bonds (where M = modifying oxides) in the glass network was not significantly altered [38]. However, the addition of Fe2O3 and/or FeO resulted in an increase in the fragility index. This result was attributed to a highly depolymerised structure, hence shorter chain lengths due to the higher 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 amount of Na2O [39] and Fe2O3 [40]. Therefore, the decrease in the fragility values with the addition of B2O3 was an indication of longer phosphate chain length, which was also crosslinked by B 3+ ions.

Fibre manufacturing
Glass formulations containing 5 mol% B2O3 (P45B5Fe3) were found to be considerably easier to fiberise than the only Fe2O3 and/or FeO (P45Fe3 and P45Fe5) containing glasses.
However, glass formulations containing 5 mol% B2O3 with Fe2O3 and/or FeO contents fixed at 3 mol% (P45B5Fe3) were found to be qualitatively easier to fiberise than the glass formulations with 5 mol% B2O3 and 5 mol% Fe2O3 (P45B5Fe5). In this study, it was found that addition of 5 mol% B2O3 allowed for fibre manufacture from glass formulations with P2O5 content fixed at 45 mol% and this fibre production was continuous with no breakage for up to 3 hours. It was difficult to draw continuous fibres from the P45B5Fe5 glass formulation as the glass viscosity of this melt was close to the maximum temperature limit for the inhouse melt-drawn fibre production system used. It was observed that at high temperature the viscosity of the glasses was too low to pull fibre and upon lowering the temperature to achieve a suitable viscosity the glass was found to crystallise in the bushing of the crucible [41]. In our study it was possible to pull fibre from glass formulations with P2O5 content fixed to 45 mol%. It has been reported that, it is difficult to pull fibre from glass formulation with no B2O3, whereas addition of only 5 mol% B2O3 to the glass formulations made the fibre pulling comparatively easier and more importantly, continuous [15]. The MAS NMR analysis conducted on the glasses investigated in a study revealed that addition of 5 and 10 mol% B2O3 to the glass formulations with P2O5 content fixed to 40, 45 and 50 mol% increased the chain length [42]. PBG formulations with P2O5 content fixed at 45 mol% were contained shorter chains due to a mixture of Q 2 and Q 1 species (with the Q 1 and Q 2 ratios reportedly to be in the range of ~21 and ~81, respectively). 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 Whereas, formulations where P2O5 content was fixed at 45 mol% and B2O3 content fixed to 5 mol% reportedly had a longer chain structure composed mostly of Q 2 species (Q 1 and Q 2 ratios were reported to be in the range of ~8 and ~ 92, respectively). Therefore, it was assumed that the addition of B2O3 to the PBG formulations increased the chain length, which made the fibre pulling process continuous. Saranti et al. also suggested that addition of boron could alter the structure of the phosphate network via the formation of long chain Q 2 species rather than Q 0 or Q 1 units [1]. Successful fiberisation largely depends on the chain length and proper adjustment of melt temperature to obtain a suitable viscosity, since it is not feasible to draw fibres from glasses with low melt viscosities using the current approach. Sharifah et al.
suggested that the decrease in fragility index corresponded to an increase in phosphate chain length and higher cationic field strength. Therefore, it is suggested that an increase in the chain length (i.e. Q 2 species) due to addition of B2O3 helped to ease manufacture of fibres from phosphate glass formulations with fixed P2O5 contents of 45mol% which was also evidenced by the decrease in fragility index as discussed above.
Replacing monovalent cation oxides with divalent or trivalent/divalent cation oxides has been shown to increase the cross-link density which eventually increased the mechanical properties (tensile fracture stress and tensile modulus) of the fibres [45]. Addition of B2O3 to the phosphate glass structure can form highly cross-linked BPO4 units which are composed of interconnected BO4 and PO4 tetrahedral units [46,47]. Moreover, the inclusion of a second network former to phosphate based glasses increased the tensile strength and elastic modulus owing to the strong interaction between chain structures and the formation of threedimensional structures [48]. As B2O3 is a natural glass network former [49][50][51], along with cross-linking, the borate ions can also participate in the formation of chain structures i.e. become a part of the backbone of the glass network.
Therefore, the improvement in fibre tensile strength with addition of B2O3 could be attributed to the fact that addition of boron to the phosphate glass network increased the cross-linking density and chain lengths by becoming or forming part of the glass network as also evidenced by higher Tg and enhanced processing window studies highlighted above.
The Weibull modulus of the fibres studied in the present study was seen to range from 7.1 to 10.2. Karabulut et al. studied the tensile strength of a series of phosphate based glass fibres drawn via the melt drawn system [52]. They found Weibull modulus values in the range between 6 and 12. The Weibull modulus (m) is a well-known and accepted method to describe the physics of fibre failure [53]. If a value of m is large, then stresses even slightly below the normalising value σo would lead to a low probability of failure. However, a low Weibull modulus would also introduce uncertainty about the strength of the fibre [25].
The tensile modulus of the P45Fe3 and P45Fe5 fibres was found to increase by ~18 % and 17% as 5 mol% B2O3 was added to the glass formulations. Similar effect of increasing B2O3 on the tensile modulus of the fibres had also been discussed by Sharmin et al. [15]. The tensile modulus of a material is an intrinsic property and depends on the field strength of the cation and the packing density of the oxygen atoms [54] as the cations with higher field strength can interact strongly with the negatively charged phosphate anions and therefore hinder mutual rotations and displacements of the anions [48]. Therefore, the increased 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 interaction between the negatively charged phosphate anions and the cations is attributed to the increased tensile modulus values of B2O3 containing fibres.
There was no statistical difference in the mechanical properties between P45B5Fe3 and P45B5Fe5 fibres and it was easier to pull P45B5Fe3 and P45B5Fe5 fibres, as compared to P45Fe3 and P45Fe5 fibres. Moreover, the mechanical properties of P45B5Fe3 and P45B5Fe5 fibres were also significantly higher than P45Fe3 and P45Fe5 fibres.