Applied Polymer ARTICLE 到 Conduction 菲:N、" Frequency(Hz) Frequeney(Hz) 1000 DOOOOO Frequency(Hz) 10000000 Frequency (Hz) Frequency(Hz) Figure 6. Isothermal runs of the dielectric permittivity(')and of the dissipation factor(tan 8)versus frequency for the(a, d)polyester matrix and its composites(b, e) NFRUP and(c,f)EGMRUP, respectively. decreased relaxation times (ts)and shifted the maximum appearance of MwS polarization, so the absence of the MWS higher frequencies peak in the tan 8 curves, as illustrated in Figure 6(f), which In the case of the NFRUP composite, in addition to the relaxation resulted in the slow enhancement of the permittivity eat low associated with the direct-current(dc) conductivity effect above frequencies and high temperatures in comparison to that of the Tg the dissipation factor curves revealed the presence of a NFRUP composite, could be explained relaxation attributed to the MWS effect. This relaxation was the The incorporation of natural fibers into the matrix also led to a result of the charge accumulations between the fibers and matrix decrease in the intensity of the dissipation factor, and this was having different conductivities and permittivities. Therefore, amplified in reverse in the E-glass fibers. We recognize that the the enhancement of these relaxations above Tg amplified the e in- measurement of the dissipation factor of insulating material is cial polarization was not visible in the case of the EGMRUP com- mpo important because the loss tangent is a measure of the alternat posite. It is also worthwhile to note that previous experimental ing-current electrical energy, which is converted to heat in an work on molecular relaxation in an insulator. Such heat raises the insulator temperature and accel- erates its deterioration so natural fibers enhance the thermal on hydroxypropyl cellulose and acrylic polymer showed that at insulation in composites low frequencies, the ionic conductivity dominated the dielectric spectra of the composite. Furthermore, Perrier revealed in a The a relaxation, which was already seen in the polyester resin study on MwS relaxations in polystyrene-A-glass bead compo- was completely masked by the dc conductivity effect in the case sites that the MWS relaxation process in composites consisting of of the EGMRUP composite and by interfacial polarization in an insulating matrix loaded with fillers made of a more conduc- the case of the NFRUP composite Indeed, the maximum of tan tive material depended on the volume fraction of the filler and 8 for the a relaxation of the matrix was 2.58 x 10-for a tem- heir size. In our case, the E-glass fibers were less conductive than perature of 90C and a frequency of 3.91 x 10 Hz. Neverthe- A-glass fibers. This proved the electrical characteristics of the less, in the case of the NFRUP and EGMRUP composites, the constitutive elements of the composite(the permittivity and isothermal runs of the dissipation factor(tan a)in the same onductivity of the matrix and fillers) to be less different for the temperature range showed an enhancement in the intensity and Www. MaterialSviewS. cOm WILEYONLINELIBRARY. COM/APP J APPL POLYM. SCL 2013, DOl: 10.1002/APP. 38499 493decreased relaxation times (ss) and shifted the maximum to higher frequencies.52 In the case of the NFRUP composite, in addition to the relaxation associated with the direct-current (dc) conductivity effect above Tg, the dissipation factor curves revealed the presence of a relaxation attributed to the MWS effect.53 This relaxation was the result of the charge accumulations between the fibers and matrix having different conductivities and permittivities.54 Therefore, the enhancement of these relaxations above Tg amplified the e0 in￾tensity as the temperature increased. It was noted that the interfa￾cial polarization was not visible in the case of the EGMRUP com￾posite. It is also worthwhile to note that previous experimental work on molecular relaxation in an anistropic composite based on hydroxypropyl cellulose and acrylic polymer showed that at low frequencies,55 the ionic conductivity dominated the dielectric spectra of the composite. Furthermore, Perrier56 revealed in a study on MWS relaxations in polystyrene–A-glass bead compo￾sites that the MWS relaxation process in composites consisting of an insulating matrix loaded with fillers made of a more conduc￾tive material depended on the volume fraction of the filler and their size. In our case, the E-glass fibers were less conductive than A-glass fibers.57 This proved the electrical characteristics of the constitutive elements of the composite (the permittivity and conductivity of the matrix and fillers) to be less different for the appearance of MWS polarization,56 so the absence of the MWS peak in the tan d curves, as illustrated in Figure 6(f), which resulted in the slow enhancement of the permittivity e0 at low frequencies and high temperatures in comparison to that of the NFRUP composite, could be explained. The incorporation of natural fibers into the matrix also led to a decrease in the intensity of the dissipation factor, and this was amplified in reverse in the E-glass fibers. We recognize that the measurement of the dissipation factor of insulating material is important because the loss tangent is a measure of the alternat￾ing-current electrical energy, which is converted to heat in an insulator. Such heat raises the insulator temperature and accel￾erates its deterioration, so natural fibers enhance the thermal insulation in composites. The a relaxation, which was already seen in the polyester resin, was completely masked by the dc conductivity effect in the case of the EGMRUP composite and by interfacial polarization in the case of the NFRUP composite. Indeed, the maximum of tan d for the a relaxation of the matrix was 2.58  103 for a tem￾perature of 90
C and a frequency of 3.91  103 Hz. Neverthe￾less, in the case of the NFRUP and EGMRUP composites, the isothermal runs of the dissipation factor (tan d) in the same temperature range showed an enhancement in the intensity and Figure 6. Isothermal runs of the dielectric permittivity (g0 ) and of the dissipation factor (tan d) versus frequency for the (a,d) polyester matrix and its composites (b,e) NFRUP and (c,f) EGMRUP, respectively. WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2013, DOI: 10.1002/APP.38499 493 ARTICLE