## 2 Methods for Adhesion Improvement
### Fiber Treatment and Matrix Modifications  
Several studies have been performed in order to determine the influence of the interphase PP/GF on the mechanical properties by modifying the fibers with \(\gamma\)-aminopropyltriethoxysilane and various polymer dispersions (PE/polyurethane), PP, PP/polyurethane and epoxy) with PP-g-MA. The experimental results showed that physical and chemical interactions established in the composite improved the interfacial strength values and mechanical properties in all cases [21; 27; 28; 29; 30; 31; 32; 33]. [PERSON] _et al._[34; 35] investigated a way to increase mechanical properties of PP/GF composites modifying the fiber/matrix interphase using model multifunctional sizing agents (triethoxy and chlorodimethyl silane grafted isotactic copolymers of propene and dienes). These polymers were synthesized through heterogeneous Ziegler-Natta catalysis copolymerization and hydrosilylation with a Speier catalyst. The resulting multifunctional sizing agents can create chemical bonds with the fiber surface and co-crystallize with the PP matrix (co-crystallization is obtained between polymers having very similar chemical and crystalline structures). They found that, for ramificated poly(propene)s it is possible to adjust the surface free energy with the silane content. The more important interfacial shear stress was obtained with poly(propene-co-8.5%methyloctadiene-g-chlorodimethylsilane) showing that a compromise must be found between the silane content and polymer crystallinity. The main disadvantage of these methods is the relationship between high-costs and low efficiency. **3. Fiber _In-Situ_ Polymerization as a Route of Adhesion Improvement  
PP polarity is so low then the adhesion with GF is bad. As it was shown above, a lot of effort was done in order to enhance the compatibility between both surfaces, but improvement obtained was low. Also its viscosity in the molten state is so high that good penetrability in GF mats is impeded. The best possible adhesion is given by chemical anchorage between the polymer and fiber. Following this idea, PP was polymerized and chemically bonded on the glass surface [36, 37] in our group. This reaction involves a metallocene catalysis starting with propylene gas. In this sense, when polymerization is performed onto the glass-fiber mat, the penetrability of this gas is so high that PP \"prepreg\" mats could be obtained [38]. The experimental reaction route involves an initial contact with methylaluminoxane (MAO) and hydroxy-\(\alpha\)-olefin (9-decen-1-ol) to generate the anchorage points on the fiber surface, followed by a propylene polymerization catalyzed by EtInd\({}_{2}\)ZrCl\({}_{2}\) (metallocene)/MAO. The reaction was carried out onto GFs of about 25 \(\upmu\)m nominal diameter, provided by Vetrotex Argentina. It should be noted that, because the extruding glass process takes place at very high temperatures on the surface, hydrolysis used for all surface modifications, silanization, titanium, and so on, and will also be used as anchor points for the proposed direct copolymerization in this study. Figure 1 shows the reaction scheme proposed for the PP grafting molecules onto glass surface. **Figure 1.** Scheme proposed for the polypropylene (PP) grafting molecules onto glass surface. The graft reaction of PP onto GFs includes two steps:  
* _Glass-MAO pretreatment_. The first step is MAO addition in order to produce an appropriate concentration of CH\({}_{3}\) groups (from the MAO), which subsequently react with the compounds to be copolymerized (the hydroxy-\(\alpha\)-olefin) through the OH groups. The glass reacts with MAO through the OH groups anchored on its surface, releasing methane (CH\({}_{4}\)) and generating a bond Al-O-Si (glass). The methyl group on the (glass) Si-O-Al-CH\({}_{3}\) reacts with the OH of the hydroxy-\(\alpha\)-olefin, releasing CH\({}_{4}\).