The water absorption (WA) and thickness swelling (TS) values for polymer composites are presented in Table 3. The WA and TS values increased with increasing hazelnut husk flour content in the composites. The highest WA and TS values, 25.03% and 8.43%, respectively, were determined in composites filled with 60% hazelnut husk flour (group code G), without the coupling agent, after 28 days. The water absorption was more pronounced in the specimens compared to the thickness swelling. The water absorption values were always higher than the swelling values. The swelling and absorption of the specimens were significantly affected by the submersion time. The lignocellulosic material is the hydrophilic component of the composites, while the polymer matrix has hydrophobic properties. Therefore, the WA and TS rates increased as the amount of hazelnut husk flour increased in the polymer composites. Moreover, the HDPE is hydrophobic in nature, and the hazelnut husk flour was not sufficiently encapsulated by the polymer matrix, R-HDPE, as its content increased in the composite. As a result, water molecules could more easily penetrate the interior of the polymer composites (Khanjanzadeh et al. 2012). The water absorption of thermoplastics filled with lignocellulosic depends on the gaps and flaws at the interfaces, micro-cracks in the matrix formed during the compounding process, fine pores, and the number of free hydroxyl groups of the lignocellulosic. The presence of voids and defects located in the filler/matrix interface was due to poor dispersion of the filler in the polymer matrix. The number of the voids and defects in the R-HDPE composite increased with increasing content of the filler. The SEM micrographs of the tensile fracture surface of the composites confirmed a loss of interfacial bonding between the filler and the polymer matrix. It is clearly shown in Fig. 1A that there was no a good surface interaction between the filler and R-HDPE matrix in the absence of MAPE. The amounts of voids and defects between the filler and polymer matrix decreased as the compatibilizer MAPE was incorporated in the composite. The strong bond between the filler and R-HDPE is shown in Figure 1B. Different from untreated composites, the fibers of hazelnut in the maleated composites had a rougher fractural surface. It indicated that a maleated copolymer indeed strengthened the interfacial adhesion through chemical bonding.
Mursit 4 (2006) crack
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The poor interfacial bonding between the hazelnut husk flour and polymer matrices increased with increasing of hazelnut husk flour content. The poor interfacial bonding creates micro-cracks. As a consequence, less energy is needed to initiate a crack in the composites, and this cause decreasing impact strength. The composites produced with the MAPE showed lower impact strength values than that of composites without the MAPE. The use of coupling agent enhanced the fiber-matrix interaction thus increasing the brittleness of the composite. Similar findings were reported in other studies (Tascioglu et al. 2014; Tufan et al. 2016). 2ff7e9595c
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