## 2 Methods for Adhesion Improvement
### Glass Fiber Treatment  
The glass surface modifications by treatment with a coupling agent are used to improve the fiber/matrix interfacial strength through both physical and chemical bonds, and to protect the fiber surface from environmental conditions, such as moisture and reactive fluids. Organofunctional silanes are the most widely used coupling agents for improvement of the interfacial adhesion in glass-reinforced materials. Their effectiveness depends on the nature and pretreatment of the substrate, the silane type, the silane layer thickness and the application process. In a relatively dry state, the proper selection of a silane coupling agent is an effective means of promoting interfacial adhesion and enhancing mechanical properties. Under wet conditions, however, its effectiveness substantially depends on the nature of the chemical bond between the silane coupling agent and the primary constituents, _i.e._, the GFs and the polymeric matrix. A variety of mechanisms have been proposed to explain the function of silanes at the interphase. [PERSON] and colleagues were the first to perform a systematic study of the effectiveness of over a hundred silane coupling agents on the wet strength of epoxy and polyester laminates [13, 14, 15, 16]. These data suggest that the primary factors are:  
* The chemical reactivity of the silane organofunctional group to form covalent bonds with the polymer matrix. * The primary or secondary chemical bond formation at the glass interface. * The ability of the polymer matrix to diffuse into the silane \"interphase\" to form a rigid, tough, water resistant interpenetrating polymer network as a transition zone between the bulk matrix and glass reinforcement. The coupling mechanism of these agents has not been fully clarified, probably because of the complex nature of the interfacial interactions [1, 5, 7, 11]. The chemical bonding theory states that silane undergoes a chemical reaction with the surface of inorganic substances to form Si-O-M bonds (where M is either a Si atom from glass or metallic atom in general). This theory was verified for silanesreacting with silica, where the alkoxysilanes undoubtedly formed Si-O-M bonds. Since glass consists mainly of SiO\({}_{2}\), silanol and siloxane groups, they are likely to be present at the GF surface. When heated, silanol groups decompose into siloxano by releasing water. Also siloxane groups, formed at moderate temperatures, will rehydrate in the presence of water, to form silanol groups. In any case, the conditions of glass surfaces depend on the environment, particularly on the moisture content. It can be expected that a few layers of free water (thickness in the range of the diameter of the water molecule) remain weakly adsorbed. In unsaturated polyesters-silane-treated GF composites, the interphase strength depends on the reactivity of the agents to the unsaturated polyesters. This fact strongly suggests the formation of chemical bonds between the resin and the coupling agents. Instead, the last ones have less effect on thermoplastic resins, in particular on polyolefins, for which there are no specific reactions that may involve silanes. Studies on silane coatings include the development of aminosilane with an unsaturated bond, carboxylic acid functional silane, cationic silane, silyl peroxide, and animimide. [PERSON] investigated the effect of different types of silane and their concentrations in the interphase chemistry of the filler particle or fiber reinforcement in a polymer matrix and their influences on the fracture properties. Surface fracture analysis was carried out to determine the level of fiber-matrix debonding. It was found that physiscorbed silane removing enhances its chemisortion in order to form bonds with the substrate and improve the fiber-matrix adhesion [17]. Other coupling agents used with GFs are titanium compounds, known as titanates.