RTICLE Applied Polymer type of fiber can complement what is lacking in the other. In our case, the NFRUP composite was a hybrid composite with two natural fibers (alfa and wool)and polymeric fibers(Pet- PE), which acted as thermobinder fibers, so the hybridization of 5.2% the NFRUP composite explained the enhancement of their ten- sile properties compared to those of the EGMRUP composit In addition, the alfa fibers were soaked for chemical extraction and bleached in an NaClo solution and this could have con- tributed to the improvement of the interaction between th fibers and the matrix. Indeed. it was observed that the interfa cial shear strength of hemp-fiber-reinforced UP composite 46.2 increased when the hemp fibers were treated with sodium hy droxide. This was explained by the greater esterification between alkali-treated hemp fibers with UP. It appeared that the removal of pectin and waxy materials from the surface of untreated hemp fibers, for alkali-treated fibers, increased the number of available OH groups for greater esterification with the UP matrix. Also, Kumar et al. developed a study on the compatibility of unbleached and bleached bamboo fibers with a linear low-density polyethylene matrix, in which they showed in each bar corres pond that the bleached fibers had more compatibility with the matrix. They demonstrated that during bleaching, some materials(lig- nin moieties, natural waxes, and pectins found in cellulose fibers, etc. )of high water adsorption capacity might have been dissolved from the delignified fiber; hence, the bleached bamboo fibers could take up less water. They found that the thermal and mechanical properties of the bleached bamboo fibers were also better than those of the unbleached bamboo fiber compo- sites; this further supported the benefit of using bleached bam- Figure 5.(a)Young's modulus,(b)tensile strength at break, and (c)stress boo fibers as reinforcement materials at break of the NFRUP and EGMRUP composites. superior specific tensile properties than the EGMRUP compos- Dielectric Properties ite, as shown in Table I. Similar results were found by Arbelaiz Comparative plots of the frequency dependence of the dielectric et al. in treated flax fiber-polypropylene(PP)composites. permittivity(a) and the dissipation factor(tan a)in the UP lany experimental investigations have been done from this per- resin matrix and its NFRUP and EGMRUP composites for dif spective to compare the mechanical properties of natural-fiber- ferent temperatures from 40 to 150C in increments of 10C ar matrix composites with E-glass-matrix composites. 6.7 Indeed, shown in Figures 6(a-f). An overall increase in a' with temper Oksman" found that the stiffness of natural-fiber-mat-rein- ture at low frequencies and a decrease of the behavior with forced thermoplastics with higher or at least the same fiber increasing frequency were observed. Also, the dielectric loss fac content was comparable with that of glass-fiber composites. tor(tan 8)displayed the presence of two relaxations, which Therefore, for rigidity applications, flax fiber bundle-PP compo- depended on the temperature and frequency, for the matrix and sites could compete with glass-PP composites. On the other its NFRUP composite and only one relaxation for the EGMRUP hand, it is known that natural-fiber-reinforced polymer matrix composite. Indeed, for the resin, these relaxations were related composites show lower modulus and strength values and poorer to an electrode polarization for low frequencies and to the glass moisture resistance than glass-fiber-reinforced composites. transition for the high frequencies when the temperature One possibility for obtaining a composite with better mechani- increased. The latter was associated with the glass-rubbery tran- cal performance is the reinforcement by two or more types in a sition of the polymer. Its relaxation peak maximum shifted to single matrix; this led to a great diversity of material proper- higher frequencies with increasing temperature because the ties. The advantage of using a hybrid composite is that one increased temperature resulted in faster movement, which led to Table L. Tensile Properties of the NFRUP and EGMRUP Composites Composi material o(MPa) o p(MPa cm /g) E(GPa) Etp(GPa cm"/g) 1219+0.61120±0.6 0.936±0.0460.92±0.05 GMRUP16.75±0.837.40±0.370.837±0.0410.37±0.02 at, p and Et are the stress at break, the density and the Young' s modulus of the composites respectively 492APPL. POLYM.Sc.2013,D0:10.1002PP38499 WILEYONLINELIBRARY. COM/APP EWILEY NONLINE LIBRARYsuperior specific tensile properties than the EGMRUP compos￾ite, as shown in Table I. Similar results were found by Arbelaiz et al.45 in treated flax fiber–polypropylene (PP) composites. Many experimental investigations have been done from this per￾spective to compare the mechanical properties of natural-fiber– matrix composites with E-glass–matrix composites.46,47 Indeed, Oksman48 found that the stiffness of natural-fiber-mat-rein￾forced thermoplastics with higher or at least the same fiber content was comparable with that of glass-fiber composites. Therefore, for rigidity applications, flax fiber bundle–PP compo￾sites could compete with glass–PP composites.47 On the other hand, it is known that natural-fiber-reinforced polymer matrix composites show lower modulus and strength values and poorer moisture resistance than glass-fiber-reinforced composites.49 One possibility for obtaining a composite with better mechani￾cal performance is the reinforcement by two or more types in a single matrix; this led to a great diversity of material proper￾ties.50 The advantage of using a hybrid composite is that one type of fiber can complement what is lacking in the other. In our case, the NFRUP composite was a hybrid composite with two natural fibers (alfa and wool) and polymeric fibers (PET– PE), which acted as thermobinder fibers, so the hybridization of the NFRUP composite explained the enhancement of their ten￾sile properties compared to those of the EGMRUP composite. In addition, the alfa fibers were soaked for chemical extraction and bleached in an NaClO solution, and this could have con￾tributed to the improvement of the interaction between the fibers and the matrix. Indeed, it was observed that the interfa￾cial shear strength of hemp-fiber-reinforced UP composite increased when the hemp fibers were treated with sodium hy￾droxide.38 This was explained by the greater esterification between alkali-treated hemp fibers with UP. It appeared that the removal of pectin and waxy materials from the surface of untreated hemp fibers, for alkali-treated fibers, increased the number of available OH groups for greater esterification with the UP matrix. Also, Kumar et al.51 developed a study on the compatibility of unbleached and bleached bamboo fibers with a linear low-density polyethylene matrix, in which they showed that the bleached fibers had more compatibility with the matrix. They demonstrated that during bleaching, some materials (lig￾nin moieties, natural waxes, and pectins found in cellulose fibers, etc.) of high water adsorption capacity might have been dissolved from the delignified fiber; hence, the bleached bamboo fibers could take up less water. They found that the thermal and mechanical properties of the bleached bamboo fibers were also better than those of the unbleached bamboo fiber compo￾sites; this further supported the benefit of using bleached bam￾boo fibers as reinforcement materials. Dielectric Properties Comparative plots of the frequency dependence of the dielectric permittivity (e0 ) and the dissipation factor (tan d) in the UP resin matrix and its NFRUP and EGMRUP composites for dif￾ferent temperatures from 40 to 150
C in increments of 10
C are shown in Figures 6(a–f). An overall increase in e0 with tempera￾ture at low frequencies and a decrease of the behavior with increasing frequency were observed. Also, the dielectric loss fac￾tor (tan d) displayed the presence of two relaxations, which depended on the temperature and frequency, for the matrix and its NFRUP composite and only one relaxation for the EGMRUP composite. Indeed, for the resin, these relaxations were related to an electrode polarization for low frequencies and to the glass transition for the high frequencies when the temperature increased. The latter was associated with the glass–rubbery tran￾sition of the polymer. Its relaxation peak maximum shifted to higher frequencies with increasing temperature because the increased temperature resulted in faster movement, which led to Figure 5. (a) Young’s modulus, (b) tensile strength at break, and (c) stress at break of the NFRUP and EGMRUP composites. Table I. Tensile Properties of the NFRUP and EGMRUP Composites Composite material rt (MPa) rt/q (MPa cm3/g) Et (GPa) Et/q (GPa cm3/g) NFRUP 12.196 0.61 12.0 6 0.6 0.936 6 0.046 0.92 6 0.05 EGMRUP 16.75 6 0.83 7.40 6 0.37 0.837 6 0.041 0.37 6 0.02 rt, q and Et are the stress at break, the density and the Young’s modulus of the composites respectively. 492 J. APPL. POLYM. SCI. 2013, DOI: 10.1002/APP.38499 WILEYONLINELIBRARY.COM/APP ARTICLE