# Spinning of Endless Bioactive Silicate Glass Fibres for Fibre Reinforcement Applications
## 1 Introduction
To ensure a stable fibre spinning process, the processing window (\(\Delta\)T) should be at least 70 K [20; 21]. Finally, a chemical coating, or sizing, is applied. Sizings are indispensable for textile fibres, regardless of their nature. These aqueous dispersions of film formers, adhesion promoters and modifiers are applied to the surface during the spinning process, where they form a protective layer. They are important for the textile processability of the fibres since they establish contact between the individual filaments, reduce roughness and thus improve the sliding properties on thread-carrying machine parts, increase strength and also enable bonding between the glass fiber and the polymer matrix. On the other hand, the silane-containing sizings are known to increase the strength of the fibres by bridging defects in the glass network [22; 23]. Obtaining fibres from bioactive silicate glasses is challenging because of the narrow processing range of many BGs. That means that these glasses tend to segregate and crystallise during preparation, making it difficult to maintain the glass in an amorphous state. The devitrification tendency can be explained by their highly disrupted silicate network with low network connectivity. Due to the high concentration of non-bridging oxygen atoms, the covalent cross-linking between the silicate chains is reduced resulting in an increased mobility of the structural units, which facilitates rearrangement to form nuclei of critical size. Overall, it can be concluded that an increasing ratio of alkali/alkaline earth metal cations increases the crystallisation tendency and, moreover, prevents a stable drawing process. Additionally, their bioactivity is lowered [2; 24]. [PERSON] et al. [25] showed that BG can be split into two groups, those that crystallise to sodium-calcium silicates such as combinete, (Na\({}_{2}\)O-2 CaO-3 SiO\({}_{2}\)) and those that crystallise to wollastonite (CaO-SiO\({}_{2}\)), with the latter having a significantly increased processing window of 300 K compared to the previous mentioned value. This shows that BG with high contents of sodium, such as 45S5, are less favourable for processing, while glasses with low contents, such as 13-93 (53 SiO2-6 Na2O-12K2O-5 MgO-20 CaO-4P2O5 wt%) [26], show reduced tendency to crystallise and are therefore easier to process. To be able to form bioactive glass fibres into textiles, they should be as thin as the technical glass fibres (4-20 um) and must have sufficient tensile strength, which, for example, is quoted as about 2000 MPa for unsized and between 2500 to 4000 MPa for sized E-glass fibres [21; 27]. A wide variety of diameters for continuous fibres made from bioactive glasses have been reported in the literature. [PERSON] et al. produced core-clad fibres from phosphate glass with diameters of 110 um and 140 um [28]. Even larger sizes were reported by [PERSON], who fabricated silicate glass fibres from 13-93 glasses and coated them with various polymers. The average thicknesses were consistently above 200 um [29]. These fibres degrade slowly over a long period of time, but are most likely not suitable for textile processing due to the large bending stiffness of such thick fibres. [PERSON] et al. showed that thin bioresorbable silicate fibres can also be produced [30]. Three glass compositions were drawn into fibres with an average thickness of 13 um by melt spinning. Strengths were exceptionally high for the bioactive glasses, with values around 2000 MPa. The dissolution behaviour was studied in Tris buffer and SBF over a period of 26 weeks. All fibre compositions studied by [PERSON] et al. [30], including the E-glass, showed significant strength loss in SBF after 26 weeks. In this work, the temperature and viscosity behaviour of four different glass systems (S53P4, 13-93, 1-06 and 18-06), whose composition was already reported by [PERSON] et al. [31; 32], were investigated and evaluated regarding their fibre spinnability.