# Surface Modification of Glass Textile for the Reinforcement of a Cement-Based Composite: A Review
## 4 Modification Techniques of Glass Fibers
### Sizing  
All commercialized fibers are covered with a thin polymeric film, named size or sizing. Typically the size is applied to freshly pulled glass fibers by means of a sizing applicator, which normally consists of a roller, positioned with its lower portion in a size reservoir 1-2 m below the bushing plate, that rotates at a fixed rate, picking up a layer of the size on its surface. Sizing formulations almost always contain an organofunctional silane, a polymeric film former, that usually make up 70-90% of the sizing and a lubricant. Organosilanes, typically organofunctional alkoxy silane, are a class of molecules that presents two moieties attached to the silicon atom: an hydrolyzable group, like a methoxy or ethoxy group, that can react  
Figure 3: Cross section of a yarn cured from a commercial avaiable ARG fabric impregnated with a SB coating. Coating content according to the producer 35%. Immage acquired with a optical microscope Leica DFC320. Figure 2: ARG roving with 6% SB coating produced at the Leibniz-Institut für Polymerforschung Dresden e. V., Germany. Image acquired with a Digital-Microscopy Keyence VHV2000. with the siliceous backbone of the glass fibers, and an organic moiety that can react with other functional groups such amino, epoxy, vinyl or other typical groups of organic polymers. Despite the great variety of organosilanes available on the market, according to [PERSON], \(\gamma\)-aminopropyltriethoxysilane (APTES), \(\gamma\)-glycidoxyspropyltrimethoxysilane (GPTMS), \(\gamma\)-methacryloxymethyltrimethoxysilane (MPTMS) and vinyltriethoxysilane (VTES) made up the majority of all the sizing employed on commercial and laboratory scale [5]. This depends on the fact that sizing formulations have been mainly designed in order to promote the interaction between the fibers and polymeric matrices, so that silane and film formers are usually chosen in order to be compatible with the intended polymer matrix as much as possible. Therefore, the range of their chemical composition reflects the range of the most common matrix employed for FRP materials. In the case of GRC, where the reinforcement already has high chemical compatibility with the matrix, the organosilane does not have a primary role as an adhesion promoter. However, also for glass fibers employed for the reinforcement of inorganic matrix, the sizing application remains a crucial manufacturing step, because organosilanes perform many other roles besides improving the bond at the fiber-to-matrix interface. Sizing is known to play a key role also on the processability and performance of glass fibers, by preventing mechanical abrasion during the spinning process, protecting the fibers against physical, chemical and environmental degradation, and maintaining the highest potential level of fiber strength [5]. Particularly the tensile strength of the fibers is reported to be increased thanks to the ability of the sizing to mitigate surface defects, inducing healing of surface flaws and preventing the possibility of creating new defects [42; 43; 44]. For what it specifically concerns the employment of glass fibers for the reinforcement of cementitious matrices, some studies suggest that optimization of the sizing formulation can lead to enhanced durability of glass fibers in an alkaline environment. [PERSON] et al. observed significant differences in the alternation of the mechanical performance of textile-reinforced concrete using commercial yarn with different sizing formulations [30]. [PERSON] et al., investigated the durability in alkaline environment of two different commercial glass fibers by immersing them in NaOH solution and observed that the residual content of sizing on the fiber, after accelerated aging, depends on both chemical formulation of the sizing and sizing content of the virgin fibers [45]. Unfortunately, studies focused on the relationship between the chemical composition of the sizing and the improvement of the fiber durability are often limited because of the unknown sizing composition of commercial fibers.