# Spinning of Endless Bioactive Silicate Glass Fibres for Fibre Reinforcement Applications
## 1 Introduction  
Glass as a transparent, smooth and chemically stable material has become an indispensable part of everyday life. By definition, glasses are solidified melts without crystallisation. When such a supercooled liquid is cooled to lower temperatures, its viscosity increases and the molecules move slower and slower until the structure is \"frozen\". This means that the time scales for molecular rearrangements are extremely long compared to the time period of experimental observation, but they can still take place. Glass is characterized by being a hard solid, but lacking an internal structure. When a substance lacks a crystal structure itis referred to as amorphous. In contrast to the regular three-dimensional arrangement of the building blocks in crystals (long-range order), in glasses there are only orders in small domains (short-range order). When heated, they therefore do not melt above a certain temperature, but soften gradually [1]. In general, glasses consist of network formers, such as silica (SiO2), boron trioxide (B2O3) or phosphorous pentoxide (P2O5), and network modifiers, which are one or several alkali and alkaline earth oxides. In addition, intermediate oxides, e.g., zirconium oxide (ZrO2), aluminum oxide (Al2O3) can be present as a third class of constituents, which then act either as network modifiers or as network formers [2]. Therefore, glass properties are determined by the composition of the glass and the manufacturing (especially the cooling rate) and are also closely associated with its surface characteristics. Typical applications, where corrosion of glass surfaces is particularly relevant, were identified by Hench & Clark [3] as follows: stability of glass containers and windows, leaching of nuclear waste encapsulants, preservation of glass antiquities, reliability of fibre optic interfaces, bonding of bioglasses to living tissues and environment sensitivity in fracture mechanics. As glass corrosion occurs, hydration, hydrolysis of the ionic covalent network, and exchange between ions, especially from the network modifiers and intermediate oxides and protons in solution, are closely related. Depending on the glass composition and on the surrounding conditions, various secondary phases may precipitate from dissolved elements. In most cases, an amorphous, hydrated layer, commonly referred to as a \"gel\", forms on the surface, with thickness ranging from nanometer range up to the macroscopic scale [4]. According to [PERSON], silicate glasses can be assigned into five categories. Class I to III glasses are considered stable systems. Class I provides the best water resistance. These glasses are almost inert and form a hydrated layer less than 5 nm thick. On class II glasses a silica-rich protective film is deposited due to selective alkali ion removal. The formation of to passivating layers is characteristic of class III glass surfaces. Glasses with additional Al2O3 or P2O5 are able to precipitate either alumina-silicate or calcium-phosphate species on top of a silica-rich layer which can be very durable. Compositions of class IV and V are designated as hydrolvtically active glasses. Surfaces of class IV glasses do not show any passivating effect. These glasses have a comparatively low silica content. In contrast, the glass systems that are most unstable to aqueous solutions dissolve faster than a gel layer can form and represent class V. Such glasses are undergoing congruent dissolution with nearly equal loss of ions [3]. Hydrotically active glasses have been in technical use for a long time. The group of water glasses are well-known representatives. These are alkaline sodium silicate solutions that react with soluble salts and salt solutions forming solids. For example, water glasses are used as adhesives and bleaching agents in the paper industry, as binders for mineral paints or as particle-free grout for sealing masonry [5]. Moreover, borosilicate glasses with a high alkali content, as well as many other borate glasses, have very good solubility.