Glass Fibers: Quo Vadis?

Since the early 1930s, the process of melting glass and subsequently forming fibers, in particular discontinuous fiber glass or continuous glass filaments, evolved into commercial-scale manufacturing. Most commonly, a direct melt process is applied. Thereby, the several raw materials were mixed, fed into a furnace, melted and forwarded to the fiber-forming units. Here, the filaments are formed by passing a bushing plate with a certain number of nozzles. Both the number of nozzles as well as its design may vary. Arrangements of 4000 or 8000 nozzles frequently occur. The bushing plate is the most important and expensive part of the machinery for glass fiber processing. The bushings are heated electrically and their temperatures are precisely controlled to maintain a constant viscosity of the glass melt. A high-speed winder catches the filaments at a circumferential speed, which is much faster than the molten glass that exits the bushings. Thus, a tension is applied to the filaments. The fiber-forming process is affected by the constitution and properties of the glass melt. Although glass fibers can even be made from strong viscous silicate melts, other ingredients are added to reduce the melting temperature and viscosity and impart other properties for specific applications. E-glass, originally aimed at electrical applications, with a composition including SiO2, AI2O3, CaO and MgO, was further developed as a more alkali-resistant and high-performance alternative to the original soda lime glass in order to design environmentally and energy-friendly glass compositions [1]. Boron was also added via B2O3 to increase the difference between the temperatures at which the E-glass batch melted, or it was avoided, motivated by environmentally-friendly aspects. S-glass fibers, developed for higher strength, are based on a SiO2-AI2O3-MgO formulation but contain higher percentages of SiO2 for applications in which tensile strength is the most important property. AR-glass fibers (alkali-resistant) for applications in textile-reinforced concrete were developed by adding Zr2O3 or basalt fibers which became popular as glass fibers rich in iron-oxide with enhanced Young’s modulus and temperature resistance compared to those of E-glass fibers. In the final stage of the fiberization, a chemical coating or sizing is applied. Sizing is typically added at 0.5 to 2.0 wt % and includes coupling agents, film formers and lubricants. The sizing constituents influence both filament tensile strength and strand integrity, cause the fiber to strengthen the adhesion at the interface depending on the resin chemistry, improve the resin wetting and the compatibility with the resin, if the filaments are used for composites. Summarizing the present state, the basic glass fiber processing as well as the glass fibers, have changed by many refinements since its commercialization more than 80 years ago, especially due to two main drivers: (i) highly economic and energy-efficient processing; (ii) improvement of the performance of the products. Thus, the manufacturers continue to push forward on both tasks, especially in their pursuit of newer applications for glass fiber reinforced composites. Through numerous process and product innovations, a number of new manufacturers contributed to a worldwide structural composite reinforcements market of roughly 4 to 5 million tons per year. Glass fibers are the most common reinforcement used in the polymer matrix, currently accounting for 98 vol % of composites in the United Kingdom and European composites production [2,3].