## 2 Methods for Adhesion Improvement
### Matrix Modifications  
Another way to improve the thermoplastic/glass fiber interphase is to add a specific modified polymer to the matrix. Taking into account that polyolefins are highly non-polar, the introduction of polar functional groups to the PP chains can improve compatibility with polar strengthening and achieve a homogenous dispersion of additives, fillers and reinforcements. The polar groups in PP chains are introduced by reactions with species that contain specific functional groups in their structures, such as ester, carboxylic acid or anhydride groups. This kind of strategy is used for PP/GF composites in which a maleic anhydride grafted polypropylene (PP-g-MA) is added in order to react with the amine group of the silanized glass surface [20,21]. A similar methodology was adopted by [PERSON] _et al._ who used oligomers of poly(sulfone) and poly(carbonate) to promote the fiber bonding with different matrices. In these cases, the coupling proceeds from the reaction between organic functions of the silanized glass surface and the modified oligomer [22]. [PERSON] _et al._[23] evaluated the ability of hydroxylated polypropylenes to improve the adhesion between pure PP and glass laminates. They found evidence for chemical bonding between PP-OH and glass surfaces with a subsequent interfacial interaction. [PERSON] and [PERSON] investigated the influence of PP-g-MA on fatigue behavior in continuous glass-fiber-reinforced polypropylene (GFRPP) composites. In the case of longitudinal tension-tension fatigue, only in a slightly higher absolute fatigue strength results of the use of PP-g-MA. In the case of off-axis (shear or transverse) fatigue, improved adhesion as a result of the use of PP-g-MA results in significantly higher fatigue strength [24].  
[PERSON] _et al._[20] investigated the influence of PP-g-MA on monotonic mechanical properties in continuous GFRPP. Three-point bending tests were performed on 0\({}^{\circ}\) and 90\({}^{\circ}\) unidirectional glass-fiber/PP laminates with various weight fractions of PP-g-MA in the PP matrix. These tests showed an increase in both longitudinal and transverse flexural strength up to 10 wt% PP-g-MA, whereas at higher PP-g-MA weight fractions, a flexural strength decrease was observed. No significant influence of PP-g-MA on composite stiffness was found. Additional mechanical tests on unidirectional glass/PP composites with 0 wt% and 10 wt% PP-g-MA showed only a small increase in fiber-dominated properties such as longitudinal tensile strength and strain, whereas composite properties that are governed by the interphase, such as transverse, shear and compressive strength, showed significant increases as a result of matrix modification and an enhanced interaction between the GFs and the PP matrix.  
[PERSON] _et al._ also investigated the characterization of a PP-g-MA as an adhesion promoter for GF composites. They found that the apparent shear strength of single-fiber model composites with PP-g-MA-modified iPP was increased compared with those with homo-iPP [25].  
It is important to note that, during PP-g-MA preparation, melted PP is subject to a grafting reaction with maleic anhydride in the presence of peroxide initiators. The use of peroxide initiators together with the required processing conditions causes the formation of products not completely homogeneous and characterized by a PP degradation, which makes them unsuitable to use as a matrix or, due to the not guaranteed constant and repeatable properties, as compatibilizing agents in the preparation of GFRPP. This fact makes PP-g-MA only attractive as an additive to blend with pure PP [26].