ARTICLE Applied Polymer adhesion with hydrophobic polymer matrices, and this is con- tion processes in both composites were identified and attributed sidered one of the limitations to some of its exterior uses. The to the glass transition of the matrix and the conductivity, relatively high prices paid for natural-fiber apparel means that respectively. A third dielectric relaxation was identified only in the high quality of natural fibers is effectively elevated for com- the NFRUP composite and was attributed to the interfacial or posite applications. Almost all natural-fiber products, which Maxwell-Wagner-Sillars(MwS) polarization, which was accred- currently include nonwoven mats made from low-cost natural ited to the accumulation of charges at the fiber-polyester resin fibers at a competitive price to glass fibers, are key to the pro- matrix interfaces. This interfacial relaxation was absent in the duction of high-performance structured natural composites. EGMRUP composite. In addition, this comparative study con- Moreover, natural fibers readily absorb moisture because they firmed that in contrast to E-glass, the natural fibers enhanced contain abundant polar hydroxide groups, which provide the the thermal insulation in the composites. To study the fiber ad natural-fiber-reinforced polymer matrix composites with a high hesion in the matrix for both composites, a dielectric study was moisture sorption level. They are also sensitive to environmen- accomplished by SEM, differential scanning calorimetry(DSC) tal conditions in the sense that their physical and mechanical properties depend on the nature of the fiber-matrix adhesion. fiber-matrix adhesion characteristics in both composites, and Indeed, the role of the matrix in a fiber-reinforced composite this led to the conclusion that for stiffness applications, NFRUP to transfer the load to the stiff fibers through shear stresses at composites could compete with EGMRUP composites as surface the interface. This process requires a good bond between the coatings for the inner part of buses polymeric matrix and the fibers. Poor adhesion at the interface EXPErimeNtal means that the full capabilities of the composite cannot be exploited. This makes it vulnerable to environmental attacks, The UP matrix used in this research work was the same as that which may cause weakness and thus reduce its life span. In used in our previous study? and was supplied by Cray Valley/ other words, insufficient adhesion between the hydrophobic Total(Sousse in Tunisia). In fact, the matrix was mixed with olymers and hydrophilic fibers results in poor mechanical the initiators methyl ethyl ketone peroxide and cobalt octanone properties in natural-fiber-reinforced polymer composites at a concentration of 1.5% w/w before the alfa fibers were intro he aforementioned properties may be improved by physical duced. The alfa fibers extracted from the plant were attacked treatments(e.g, cold plasma treatment, corona treatment)and chemically by an NaOH solution and were bleached in an chemical treatments (e.g, maleic anhydride, organosilanes, iso- NaClO solution. Then, they were separated mechanically with a cyanates, sodium hydroxide, permanganate, and peroxide treat- Shirley analyzer(Ksar Hellal in Tunisia). Because the elabora ments).16, 17 Indeed, it has been shown that the pretreatment of tion of nonwoven fibers with only alfa fibers were not possible the natural fibers with chemical methods(e.g, the use of cou- because of the noncohesion between them, the latter were pling agents, e.g., silane compounds) enhances the adhesion at mixed with wool fibers to ensure cohesion. Natural fibers on the fiber-matrix interface and reduces the moisture sorption of their own cannot be thermoformed and require the addition of these fibers. As a result, the retention of natural-fiber-reinforced polymeric fibers to act as binders, so the added thermobinder polymer matrix composites under environmental aging in the fibers were composed of PET and PE as a cover. The obtained mechanical properties is improved. In addition, the amount of sheet of nonwoven fibers was made up of these three kinds of noisture sorption can be reduced significantly through the fibers. The diameters of the alfa and wool fibers were 204.86 replacement of natural fibers with a small amount of synthetic and 37.21 um, respectively. The length of the thermobinder fibers,such as glass or carbon. 18 Many techniques have been fibers was 51 mm; its count was 4 deniers, and their melting ed to provide evidence for the effect of these treatments on temperatures were 260 and 110C, respectively. The relative vol the fiber-matrix interfacial adhesion. The ones mostly used are ume fractions of these fibers in the composite NERUP had a ra dynamic mechanical analysis and scanning electron microscopy tio of alfa to wool to PET-PE of 17: 1: 2. To prepare the sheet of SEM)observation. However, alternative methods that are able nonwoven to discover the interface and help to better adapt the appropri lowed. First, the alfa, wool, and polymeric fibers(PET-PE)were ate coupling agent according to the fiber's surface and the separately cleaned and opened with an industrial bale opener matrix are still attracting much attention. Accordingly, many (two passes). Afterward, to improve blending, two other pas- experimental studies have pointed out the significance of dielec- sages through the bale opener were necessary. Next, the fibers tric spectrometry as an additional technique that can be used to were combed into a web by a carding machine, which was probe the composite interface and investigate the effect of fiber rotating drum or series of drums covered in fine wires. Finally, treatment on the evoluti because the obtained web, whose fibers were unbonded in form propertie of composites interfacial had little strength, it had to be consolidated in some way. In our case, two types of consolidation were used, the first of In this study, nonwoven alfa, wool, and thermobinder fibers which was a mechanical bonding with a needle punch. Hence, Ipoly(ester terephthalate)(PET)-polyethylene(PE)]-reinforced the web was strengthened by interfiber friction as a result of the UP resin composite [natural-fiber-reinforced unsaturated poly- physical entanglement of the fibers by needles. We used a labo- ester(NFRUP) were prepared, and their dielectric properties ratory needle-punching machine(two passes were needed). The were compared with conventionally reinforced E-glass-mat-rein- second consolidation of the nonwoven fiber sheet forced unsaturated polyester(EGMRUP). Two common relaxa- one. Indeed, the presence of the thermobinder 488 J. APPL. POLYM.Sc.2013,D0:10.1002PP38499 WILEYONLINELIBRARY. COM/APP EWILEY NONLINE LIBRARYadhesion with hydrophobic polymer matrices, and this is con￾sidered one of the limitations to some of its exterior uses. The relatively high prices paid for natural-fiber apparel means that the high quality of natural fibers is effectively elevated for com￾posite applications. Almost all natural-fiber products, which currently include nonwoven mats made from low-cost natural fibers at a competitive price to glass fibers, are key to the pro￾duction of high-performance structured natural composites.3 Moreover, natural fibers readily absorb moisture because they contain abundant polar hydroxide groups, which provide the natural-fiber-reinforced polymer matrix composites with a high moisture sorption level.4 They are also sensitive to environmen￾tal conditions in the sense that their physical and mechanical properties depend on the nature of the fiber–matrix adhesion. Indeed, the role of the matrix in a fiber-reinforced composite is to transfer the load to the stiff fibers through shear stresses at the interface. This process requires a good bond between the polymeric matrix and the fibers. Poor adhesion at the interface means that the full capabilities of the composite cannot be exploited. This makes it vulnerable to environmental attacks, which may cause weakness and thus reduce its life span. In other words, insufficient adhesion between the hydrophobic polymers and hydrophilic fibers results in poor mechanical properties in natural-fiber-reinforced polymer composites. The aforementioned properties may be improved by physical treatments (e.g., cold plasma treatment, corona treatment) and chemical treatments (e.g., maleic anhydride, organosilanes, iso￾cyanates, sodium hydroxide, permanganate, and peroxide treat￾ments).16,17 Indeed, it has been shown that the pretreatment of the natural fibers with chemical methods (e.g., the use of cou￾pling agents, e.g., silane compounds) enhances the adhesion at the fiber–matrix interface and reduces the moisture sorption of these fibers. As a result, the retention of natural-fiber-reinforced polymer matrix composites under environmental aging in the mechanical properties is improved. In addition, the amount of moisture sorption can be reduced significantly through the replacement of natural fibers with a small amount of synthetic fibers, such as glass or carbon.18 Many techniques have been used to provide evidence for the effect of these treatments on the fiber–matrix interfacial adhesion. The ones mostly used are dynamic mechanical analysis and scanning electron microscopy (SEM) observation. However, alternative methods that are able to discover the interface and help to better adapt the appropri￾ate coupling agent according to the fiber’s surface and the matrix are still attracting much attention. Accordingly, many experimental studies have pointed out the significance of dielec￾tric spectrometry as an additional technique that can be used to probe the composite interface and investigate the effect of fiber treatment on the evolution of composites’ interfacial properties.12,13,19,20 In this study, nonwoven alfa, wool, and thermobinder fibers [poly(ester terephthalate) (PET)–polyethylene (PE)]-reinforced UP resin composite [natural-fiber-reinforced unsaturated poly￾ester (NFRUP)] were prepared, and their dielectric properties were compared with conventionally reinforced E-glass-mat-rein￾forced unsaturated polyester (EGMRUP). Two common relaxa￾tion processes in both composites were identified and attributed to the glass transition of the matrix and the conductivity, respectively. A third dielectric relaxation was identified only in the NFRUP composite and was attributed to the interfacial or Maxwell–Wagner–Sillars (MWS) polarization, which was accred￾ited to the accumulation of charges at the fiber–polyester resin matrix interfaces. This interfacial relaxation was absent in the EGMRUP composite. In addition, this comparative study con￾firmed that in contrast to E-glass, the natural fibers enhanced the thermal insulation in the composites. To study the fiber ad￾hesion in the matrix for both composites, a dielectric study was accomplished by SEM, differential scanning calorimetry (DSC), and tensile testing analysis. All of these analyses revealed similar fiber–matrix adhesion characteristics in both composites, and this led to the conclusion that for stiffness applications, NFRUP composites could compete with EGMRUP composites as surface coatings for the inner part of buses. EXPERIMENTAL Materials The UP matrix used in this research work was the same as that used in our previous study20 and was supplied by Cray Valley/ Total (Sousse in Tunisia). In fact, the matrix was mixed with the initiators methyl ethyl ketone peroxide and cobalt octanone at a concentration of 1.5% w/w before the alfa fibers were intro￾duced. The alfa fibers extracted from the plant were attacked chemically by an NaOH solution and were bleached in an NaClO solution. Then, they were separated mechanically with a Shirley analyzer (Ksar Hellal in Tunisia). Because the elabora￾tion of nonwoven fibers with only alfa fibers were not possible because of the noncohesion between them, the latter were mixed with wool fibers to ensure cohesion. Natural fibers on their own cannot be thermoformed and require the addition of polymeric fibers to act as binders, so the added thermobinder fibers were composed of PET and PE as a cover. The obtained sheet of nonwoven fibers was made up of these three kinds of fibers. The diameters of the alfa and wool fibers were 204.86 and 37.21 lm, respectively. The length of the thermobinder fibers was 51 mm; its count was 4 deniers, and their melting temperatures were 260 and 110
C, respectively. The relative vol￾ume fractions of these fibers in the composite NFRUP had a ra￾tio of alfa to wool to PET–PE of 17:1:2. To prepare the sheet of nonwoven fibers (alfa þ wool þ PET–PE), four steps were fol￾lowed. First, the alfa, wool, and polymeric fibers (PET–PE) were separately cleaned and opened with an industrial bale opener (two passes). Afterward, to improve blending, two other pas￾sages through the bale opener were necessary. Next, the fibers were combed into a web by a carding machine, which was a rotating drum or series of drums covered in fine wires. Finally, because the obtained web, whose fibers were unbonded in form, had little strength, it had to be consolidated in some way. In our case, two types of consolidation were used, the first of which was a mechanical bonding with a needle punch. Hence, the web was strengthened by interfiber friction as a result of the physical entanglement of the fibers by needles. We used a labo￾ratory needle-punching machine (two passes were needed). The second consolidation of the nonwoven fiber sheet was a thermal one. Indeed, the presence of the thermobinder fibers in the ARTICLE 488 J. APPL. POLYM. SCI. 2013, DOI: 10.1002/APP.38499 WILEYONLINELIBRARY.COM/APP