《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_glass fiber-18 Comparative Study of the Dielectric Properties of Natural-Fiber–Matrix Composites and E-Glass–Matrix Composites

Applied Polymer E Comparative Study of the Dielectric Properties of Natural- Fiber-Matrix Composites and E-Glass-Matrix Composites A. Triki, M. Guicha, 2 Med Ben Hassen, 2 M. Arous1 2Laboratoire de Recherche Textile, Institut Superieure des Etudes Technologiques Ksar Hellal, Avenue Hadj Ali Soua, BP 68 unisia lAboratoire des Materiaux Composites, Ceramique et Polymeres, Faculte des Sciences de Sfax, Route de Soukra, 3018, Tur sar Hella 5070. Tunisia Correspondence to: A. Triki (E-mail: trikilamacop @yahoo. fr) ABSTRACT: In this work, we undertook a comparative study of the dynamic dielectric analysis of two composites: natural-fiber-rein- forced unsaturated polyester(NFRUP) and E-glass-mat-reinforced unsaturated polyester(EGMRUP) In both composites, two com- mon relaxation processes were identified, the first of which was the a-mode relaxation associated with the glass transition of the ma- trix. The second one was associated with conductivity that occurred because of the carriers' charge diffusion and was observed at temperatures above the glass transition and at low frequencies. However, the interfacial or Maxwell-Wagner-Sillars polarization was noticed only in the NFRUP composite. This dielectric study also revealed that compared to E-glass fibers, natural fibers enhanced the thermal insulation in the composite. Also, the study of the fiber adhesion in the matrix with scanning electron microscopy, differen tial scanning calorimetry, and tensile testing revealed a great compatibility of the fibers with the matrix in both composites. o 2012 Wiley Periodicals, Inc. J. AppL. Polym. Sci. 129: 487-498, 2013 KEYWORDS: composites; microscopy; matrix; reinforcement; relaxation Received 18 February 2012; accepted 21 August 2012: published online 22 November 2012 Do:10.1002/app38499 INTRODUCTION ubstitutes for synthetic-fiber-reinforced composites Nowadays,natural fibers such as hemp, kenaf, flax, and hene- used in the construction industry because the reinforcement is quen have gained growing interest, and their study has created low in cost and derived from renewable resources. fascinating perspectives in several applications. In fact, they The most prominent natural fibers used in structural compo- have been increasingly used as reinforcements with polymers sites are plant fibers because of their specific strength and stiff- because they have benefits compared with conventional syn- ness compared with that of glass fibers and their accessibility. thetic reinforcements, such as glass fibers, that power this grow- Among these fibers, we can cite alfa, which is the Arabic name ing interest. These benefits include not only environmental of the esparto grass or Stipa tenacissima plant. It is widely culti- and health concerns but also the sustainability of material vated in the dry region of North Africa and especially in the resources center of Tunisia, where it covers about 3500 km with an ar Among the industries that have taken advantage of natural-fiber al production of 60,000 tons. A great deal of recent research composites is the automobile industry, which has been leading work has been carried out to promote alfa fibers in reinforced he way in the use of such composites. Actually, the lower mass polymeric systems, -14 which can be applied in automotive density of natural fibers (1.4-1.5 g/cm for flax and hemp fib industry. Unsaturated polyester(UP)is a popular 2.5 g/cm' for glass fibers)leads to a reduction in vehicle polymer resin used in conventional glass-fiber-reinforced poly weight, which, in turn, leads to a reduction in fuel consump- mer composites. It has been widely used because of its excellent tion. The use of natural-fiber composites in automobile inter- processability and fast crosslinking reaction on the one hand rs also brings about many other benefits, including an increase and its good mechanical and chemical properties when fully n safety(the fractures of natural-fiber composites are not as cured on the other. 5 sharp as glass-fiber composites) and improved comfort due to Of course, the growth of natural-fiber composites cannot be the better acoustic and thermal insulation provided by the natu- carried out without any challenge. Indeed, the hydrophilic na- fibers' Moreover, natural-fiber-reinforced composites are ture of natural fibers is a potential cause of poor interfacial o 2012 Wiley Periodicals, Inc M Www. MaterialSviewS. cOm WILEYONLINELIBRARY. COM/APP J APPL POLYM. SCL 2013, DOl: 10.1002/APP. 38499 487

Comparative Study of the Dielectric Properties of Natural-Fiber–Matrix Composites and E-Glass–Matrix Composites A. Triki,1 M. Guicha,2 Med Ben Hassen,2 M. Arous1 1Laboratoire des Materiaux Composites, C
eramiques et Polyme
`res, Faculte des Sciences de Sfax, Route de Soukra, 3018, Tunisia
2Laboratoire de Recherche Textile, Institut Superieure des Etudes Technologiques Ksar Hellal, Avenue Hadj Ali Soua, BP 68,
Ksar Hellal 5070, Tunisia Correspondence to: A. Triki (E-mail: trikilamacop@yahoo.fr) ABSTRACT: In this work, we undertook a comparative study of the dynamic dielectric analysis of two composites: natural-fiber-rein￾forced unsaturated polyester (NFRUP) and E-glass-mat-reinforced unsaturated polyester (EGMRUP). In both composites, two com￾mon relaxation processes were identified, the first of which was the a-mode relaxation associated with the glass transition of the ma￾trix. The second one was associated with conductivity that occurred because of the carriers’ charge diffusion and was observed at temperatures above the glass transition and at low frequencies. However, the interfacial or Maxwell–Wagner–Sillars polarization was noticed only in the NFRUP composite. This dielectric study also revealed that compared to E-glass fibers, natural fibers enhanced the thermal insulation in the composite. Also, the study of the fiber adhesion in the matrix with scanning electron microscopy, differen￾tial scanning calorimetry, and tensile testing revealed a great compatibility of the fibers with the matrix in both composites. VC 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 129: 487–498, 2013 KEYWORDS: composites; microscopy; matrix; reinforcement; relaxation Received 18 February 2012; accepted 21 August 2012; published online 22 November 2012 DOI: 10.1002/app.38499 INTRODUCTION Nowadays, natural fibers such as hemp, kenaf, flax, and hene￾quen have gained growing interest, and their study has created fascinating perspectives in several applications. In fact, they have been increasingly used as reinforcements with polymers because they have benefits compared with conventional syn￾thetic reinforcements, such as glass fibers, that power this grow￾ing interest.1,2 These benefits include not only environmental and health concerns but also the sustainability of material resources. Among the industries that have taken advantage of natural-fiber composites is the automobile industry, which has been leading the way in the use of such composites. Actually, the lower mass density of natural fibers (1.4–1.5 g/cm3 for flax and hemp fibers vs 2.5 g/cm3 for glass fibers) leads to a reduction in vehicle weight, which, in turn, leads to a reduction in fuel consump￾tion. The use of natural-fiber composites in automobile interi￾ors also brings about many other benefits, including an increase in safety (the fractures of natural-fiber composites are not as sharp as glass-fiber composites) and improved comfort due to the better acoustic and thermal insulation provided by the natu￾ral fibers.3 Moreover, natural-fiber-reinforced composites are used as substitutes for synthetic-fiber-reinforced composites used in the construction industry because the reinforcement is low in cost and derived from renewable resources.4 The most prominent natural fibers used in structural compo￾sites are plant fibers because of their specific strength and stiff￾ness compared with that of glass fibers and their accessibility.5–7 Among these fibers, we can cite alfa, which is the Arabic name of the esparto grass or Stipa tenacissima plant. It is widely culti￾vated in the dry region of North Africa and especially in the center of Tunisia, where it covers about 3500 km2 with an an￾nual production of 60,000 tons.8 A great deal of recent research work has been carried out to promote alfa fibers in reinforced polymeric systems,9–14 which can be applied in automotive industry. Unsaturated polyester (UP) is a popular thermosetting polymer resin used in conventional glass-fiber-reinforced poly￾mer composites. It has been widely used because of its excellent processability and fast crosslinking reaction on the one hand and its good mechanical and chemical properties when fully cured on the other.15 Of course, the growth of natural-fiber composites cannot be carried out without any challenge. Indeed, the hydrophilic na￾ture of natural fibers is a potential cause of poor interfacial VC 2012 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2013, DOI: 10.1002/APP.38499 487

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 LIBRARY

adhesion 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

Applied Polymer ARTICLE sheet allowed the adhesion between fibers, which were under Tensile Testing. Tensile testing of the NFRUP and EGMRUP adapted temperature conditions. For this reason, the sheet was composites was carried out with a Lloyds Dynamometer univer- deposited in an air oven at a temperature of 120C, and the sal testing machine as per NF T 57-301 at a crosshead speed of sheet was passed through. The PET-PE fibers were submitted to 5 mm/min and a gripping length of 100 mm(Ksar Hellal in an increase in temperature. The binding was accomplished by a Tunisia). The specimen was cut out in the direction of ombination of heating, flowing, and cooling. Indeed, the initial nonwoven production. All of the results were calculated as the calorific energy weakened the outer surface of the thermobinder average of 10 samples for each test. which increased the contact surface with the other fibers, and Dielectric Analysis. Dielectric measurements were conducted then, the supplementary energy turned the outer binder into a with an Alpha dielectric-impedance analyzer(Novocontrol,Sfax fluid, whichin turn, molded the natural fibers with the ther- in Tunisia), with the measurements of the studied samples taken mobinder ones. After this consolidation. the nonwoven fibers over the temperature range from the ambient to 150C and in a were calendered at 120oC in an industry of developing nonwo. frequency interval from 10- to 10 Hz. A circular gold elec- ven materials to decrease the thickness of the sheet. The E-glass trode (2 cm in diameter)was sputtered on both surfaces of the fibers are supplied from a Tunisian company named(STIA: sample to ensure good electrical contact with the gold-plated Societe tunisienne de industrie automobile. Sousse in Tuni- measuring electrodes. A sinusoidal voltage was applied to create sia). They were presented in a sheet in which the fibers, whose an alternating electric field that produced polarization in the average length was about 50 mm, were randomly oriented. field but had a phase angle shift(8). This p a voltage with the ngle shift was measured by the comparison of the applied The composites(NFRUP, EGMRUP)were manufactured with the measured current, which was divided into capacitive and con- classical contact mold method. In fact, the fibers were deposited ductive components. With the following equations, the dielec- on the mold and impregnated with the liquid resin mixed with tric parameters were calculated suitable proportions of methyl ethyl ketone peroxide and cobalt ctanone as hardener and catalyst, respectively. The saturated materials were then pressed by a roller to remove bubbles. After the hardness of the resin was measured, the composites were withdrawn from the mold. The obtained NFrUP and EGMRUP composites had 5.2 and 5.5% fiber volume fractions, respectively (3) Measurements SEM. The morphologies of the composite surfaces were where()=-1; e and &"are the real and imaginary parts of the observed at room temperature by a Philips XL30 SEM instru- complex permittivity (e*);tan8(=g/e)is the dissipation factor; ment. A gold coating of a few nanometers in thickness was A and d are the area and thickness, respectively, of the sample; C formed on the surfaces of the samples to prevent charging, and is the capacitance; G is the conductance; and Eo is the permittivity the surfaces were examined at an accelerating voltage of 20 kV. of the free space and is equal to 8.854 x 10F/m These observations were conducted on the upper surface and Two kinds of dielectric experiments were carried out. One of he cross-sectional surface of the composite so that the longitu- them was an isochronal run with fixed frequencies and various dinal and cross-sectional aspects of the fibers, respectively, could be observed For this reason, the sample was cut with a saw at temperatures from ambient to 150C with a heating rate of room temperature in the direction of the composite cross-sec- 2C/min in a nitrogen atmosphere. The second was an isother- tional surface and perpendicular to the fiber axis. mal run with fixed temperatures and scanning frequencies from DSC. DSC was used to evaluate the glass-transition tempera cure(Ty)of the matrix and its NFRUP and EGMRUP compo- RESULTS AND DISCUSSION sites. Samples weighing between 10 and 15 mg were placed in a SEM Observation hermetic pan and sealed. A Jade DSC instrument(Perkin Elmer) SEM observation was made to explore the fiber-matrix interface was operated in the temperature interval-50 to 150.C in a in the NFRUP and EGMRUP composites. The SEM micro- nitrogen environment purged at 20 mw and according to a graphs of the composites are displayed in Figures 1(a-d)and heating-cooling-heating cycle. In the first heating step, the sam- 2(a-d). Figure 1(a, b) shows the micrographs of the longitudinal ples were heated from-50 to 150C at a heating rate of 5C/ fiber aspects for the NFRUP composite. These micrographs min. Then they were cooled from 150 to -50C at a cooling illustrate that the fibers were randomly dispersed in the matrix rate of 5%C/min. In the second heating step, the samples were and that the individual separation of the fibers were not in the heated from -50 to 150C at a heating rate of 5 C/min. The form of single fibers. Figure 1(c, d)shows the micrographs of thermograms were analyzed to estimate the Te values of the the longitudinal fiber aspects for the EGMRUP composite. This esin and its composites. The Ts value of each sample was deter- figure shows that the glass fibers were identical and were in the mined from the midpoint value of the jump in heat flow in the form of single fibers because they were synthetic. The analysis second heating run. The construction of the lines was done by of Figure 1(b, d)illustrated some physical contacts between the Pyris software, Perkin Elmer from Courtaboeuf, France. fibers and the matrix. Figure 2(a-d) depicts the micrographs of M Www. MaterialSviewS. cOm WILEYONLINELIBRARY. COM/APP J APPL POLYM. SCL 2013, DOl: 10.1002/APP. 38499 489

sheet allowed the adhesion between fibers, which were under adapted temperature conditions. For this reason, the sheet was deposited in an air oven at a temperature of 120
C, and the sheet was passed through. The PET–PE fibers were submitted to an increase in temperature. The binding was accomplished by a combination of heating, flowing, and cooling. Indeed, the initial calorific energy weakened the outer surface of the thermobinder fibers (PE fibers with a low melting temperature of 110
C), which increased the contact surface with the other fibers, and then, the supplementary energy turned the outer binder into a fluid, which, in turn, molded the natural fibers with the ther￾mobinder ones. After this consolidation, the nonwoven fibers were calendered at 120
C in an industry of developing nonwo￾ven materials to decrease the thickness of the sheet. The E-glass fibers are supplied from a Tunisian company named (STIA: Soci
et
e Tunisienne de l’Industrie Automobile, Sousse in Tuni￾sia). They were presented in a sheet in which the fibers, whose average length was about 50 mm, were randomly oriented. Composite Processing The composites (NFRUP, EGMRUP) were manufactured with the classical contact mold method.21 In fact, the fibers were deposited on the mold and impregnated with the liquid resin mixed with suitable proportions of methyl ethyl ketone peroxide and cobalt octanone as hardener and catalyst, respectively. The saturated materials were then pressed by a roller to remove bubbles. After the hardness of the resin was measured, the composites were withdrawn from the mold. The obtained NFRUP and EGMRUP composites had 5.2 and 5.5% fiber volume fractions, respectively. Measurements SEM. The morphologies of the composite surfaces were observed at room temperature by a Philips XL30 SEM instru￾ment. A gold coating of a few nanometers in thickness was formed on the surfaces of the samples to prevent charging, and the surfaces were examined at an accelerating voltage of 20 kV. These observations were conducted on the upper surface and the cross-sectional surface of the composite so that the longitu￾dinal and cross-sectional aspects of the fibers, respectively, could be observed. For this reason, the sample was cut with a saw at room temperature in the direction of the composite cross-sec￾tional surface and perpendicular to the fiber axis. DSC. DSC was used to evaluate the glass-transition tempera￾ture (Tg) of the matrix and its NFRUP and EGMRUP compo￾sites. Samples weighing between 10 and 15 mg were placed in a hermetic pan and sealed. A Jade DSC instrument (PerkinElmer) was operated in the temperature interval 50 to 150
C in a nitrogen environment purged at 20 mW and according to a heating–cooling–heating cycle. In the first heating step, the sam￾ples were heated from 50 to 150
C at a heating rate of 5
C/ min. Then they were cooled from 150 to 50
C at a cooling rate of 5
C/min. In the second heating step, the samples were heated from 50 to 150
C at a heating rate of 5
C/min. The thermograms were analyzed to estimate the Tg values of the resin and its composites. The Tg value of each sample was deter￾mined from the midpoint value of the jump in heat flow in the second heating run. The construction of the lines was done by Pyris software, Perkin Elmer from Courtaboeuf, France. Tensile Testing. Tensile testing of the NFRUP and EGMRUP composites was carried out with a Lloyds Dynamometer univer￾sal testing machine as per NF T 57-301 at a crosshead speed of 5 mm/min and a gripping length of 100 mm (Ksar Hellal in Tunisia).22 The specimen was cut out in the direction of nonwoven production. All of the results were calculated as the average of 10 samples for each test. Dielectric Analysis. Dielectric measurements were conducted with an Alpha dielectric–impedance analyzer (Novocontrol, Sfax in Tunisia), with the measurements of the studied samples taken over the temperature range from the ambient to 150
C and in a frequency interval from 101 to 106 Hz. A circular gold elec￾trode (2 cm in diameter) was sputtered on both surfaces of the sample to ensure good electrical contact with the gold-plated measuring electrodes. A sinusoidal voltage was applied to create an alternating electric field that produced polarization in the sample, which oscillated at the same frequency as the electric field but had a phase angle shift (d). This phase angle shift was measured by the comparison of the applied voltage with the measured current, which was divided into capacitive and con￾ductive components. With the following equations,23 the dielec￾tric parameters were calculated: e ¼ e 0 je 00 (1) e 0 ¼ cpðsampleÞd e0A (2) e 00 ¼ GðsampleÞd xe0A (3) where (j)2 ¼ 1; e0 and e00 are the real and imaginary parts of the complex permittivity (e*); tan d (¼e00/e0 ) is the dissipation factor; A and d are the area and thickness, respectively, of the sample; Cp is the capacitance; G is the conductance; and e0 is the permittivity of the free space and is equal to 8.854  1012 F/m. Two kinds of dielectric experiments were carried out. One of them was an isochronal run with fixed frequencies and various temperatures from ambient to 150
C with a heating rate of 2
C/min in a nitrogen atmosphere. The second was an isother￾mal run with fixed temperatures and scanning frequencies from 101 to 106 Hz. RESULTS AND DISCUSSION SEM Observation SEM observation was made to explore the fiber–matrix interface in the NFRUP and EGMRUP composites. The SEM micro￾graphs of the composites are displayed in Figures 1(a-d) and 2(a-d). Figure 1(a,b) shows the micrographs of the longitudinal fiber aspects for the NFRUP composite. These micrographs illustrate that the fibers were randomly dispersed in the matrix and that the individual separation of the fibers were not in the form of single fibers. Figure 1(c,d) shows the micrographs of the longitudinal fiber aspects for the EGMRUP composite. This figure shows that the glass fibers were identical and were in the form of single fibers because they were synthetic. The analysis of Figure 1(b,d) illustrated some physical contacts between the fibers and the matrix. Figure 2(a–d) depicts the micrographs of WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2013, DOI: 10.1002/APP.38499 489 ARTICLE

RTICLE Applied Polymer (a) (d) matrix Figure 1. SEM micrographs of the longitudinal fibers aspects for the composites: (a)100 am of NFRUP, (b)20 um of NFRUR, ()100 km of(d) EGMRUR, and(d)20 um of EGMRUP. the polished cross sections of the NFRUP and EGMRUP com posites. Figure 2(a)shows the cross section of a natural fiber, and Figure 2(c) shows the double aspects of the glass fibers, which were randomly dispersed in the matrix, that is, longitudi nal fibers aspects and fibers cross-sectional aspects. In Figure 2(b), a closer observation of the interface in the micrographs of he NFRUP composite proved better close contact between the the matrix than that in the case of the up film filled with palm tree fibers studied by Adami et al. In addition, Figure 2(b)reveals a tiny and narrow gap around the fiber that may have eventually led to cracking in the NFRUP composite Figure 2 SEM micrographs of the polished cross sections for the compo- because fatigue crack propagation resistance depends on the sites: (a m of NFRUR, (b) 10 um of NFRUP, (c) 100 um of of egmrup 490 J. APPL. POLYM.Sc.2013,D0:10.1002PP38499 WILEYONLINELIBRARY. COM/APP EWILEY NONLINE LIBRARY

the polished cross sections of the NFRUP and EGMRUP com￾posites. Figure 2(a) shows the cross section of a natural fiber, and Figure 2(c) shows the double aspects of the glass fibers, which were randomly dispersed in the matrix, that is, longitudi￾nal fibers aspects and fibers cross-sectional aspects. In Figure 2(b), a closer observation of the interface in the micrographs of the NFRUP composite proved better close contact between the fibers and the matrix than that in the case of the UP film filled with palm tree fibers studied by Kadami et al.24 In addition, Figure 2(b) reveals a tiny and narrow gap around the fiber that may have eventually led to cracking in the NFRUP composite because fatigue crack propagation resistance depends on the matrix–fiber adhesion.25–27 However, Azimi et al.25 reported Figure 1. SEM micrographs of the longitudinal fibers aspects for the composites: (a) 100 lm of NFRUP, (b) 20 lm of NFRUP, (c) 100 lm of EGMRUP, and (d) 20 lm of EGMRUP. Figure 2. SEM micrographs of the polished cross sections for the compo￾sites: (a) 100 lm of NFRUP, (b) 10 lm of NFRUP, (c) 100 lm of EGMRUP, and (d) 10 lm of EGMRUP. 490 J. APPL. POLYM. SCI. 2013, DOI: 10.1002/APP.38499 WILEYONLINELIBRARY.COM/APP ARTICLE

Applied Polymer ARTICLE would lead to an even stronger reduction in the matrix mobil ity, which is expressed by a stronger shift in Tg. The different behaviors of th es provided fur the influence of the chemical functionalization of the surface on the interfacial adhesion between the nanotubes and the epoxy Tensile Properties The interface adhesion between the reinforcing fiber and the matrix markedly influenced the mechanical performance of the composites. This region is the site synergy in composite materials, as the stress redistribution from the matrix to the fibers takes place through their bond/interphase. Therefore, although the interface region appeared to have an insignificant hatrix and its composites (NFRUP volume fraction, its influence on the overall material properties and EGMRUP) in the second was significant. Indeed, it has been observed that the exis tence of this interface/into terphase brings about significant that hybrid composites showed significant improvement in changes in the relaxation behavior of Te as discussed earlier. 42. 3 fatigue crack propagation resistance, and this could support the Figure 4 illustrates the traction curves of the NERUP and NFRUP hybrid composite, which contained two natural fibers EGMRUP composites. Both curves showed a plateau at the be lfa and wool)and the polymeric fibers(PET-PE). a closer ginning for a low strength attributed to the slide of the speci- look at the E-glass-fiber cross-sectional aspects depicted in Fig- men in the grips of the dynamometer. After this plateau,the ure 2(d) demonstrated a similar close contact in the interfacial curves exhibited a typical mechanical behavior in each compos- region between the fibers and the matrix compared with that of ite. On the NERUP composite traction curve, a depression was the nyrup composite bserved, which could be explained by the stretching of some fibers. Indeed, the tensile properties of the alfa fibers were lower The UP resin matrix and its NERUP and EGMRUP composites than those of the E-glass fibers,and this explained the absence were subjected to DSC to evaluate their thermal properties. The of such a depression on the EGMRUP composite traction curve thermographs of all of the samples are shown in Figure 3. The Accordingly, Figure 5(a-c)shows the Young's modulus, strengt T value of each sample was determined from the midpoint and stress at break of the NeRUP and EGMRUP composites value of the jump in heat flow after the second heating run. From these histograms, we observed that the tensile properties Also, the line construction was done by the Pyris software, and of the NFRUP composite were slightly better than those of he obtained values were about 68, 78, and 54C for the resin EGMRUP with regard to the obtained values of the young's and the NFRUP and the EGMRUP composites, respectively. modulus and tensile strength at break values. This showed, These results indicate that To which was attributed to the tran- according to the Young's modulus, a good cohesion of the sition from a glasslike form to a rubbery and flexible for materials to transfer stress from the matrix to the fiber How increased with the addition of the natural fibers to the matrix. ever, the stress at break of the EGMRUP composite was superior However, this temperature decreased with the addition of to that of the NFRUP composite; this could be attributed to the E-glass fibers to the matrix. Changes in the glass-transition tem- greater strength of the E-glass fibers compared to that of the perature(ATg) can indicate altered polymer chain mobility. natural fibers, as mentioned previously. When specific proper- Indeed, depending on the strength of the interaction between ties were compared, the differences in the tensile performance the polymer and the filler, this region can have a higher or became more marked. In fact, the NFRUP composite exhibited lower mobility than the bulk material, and this can result in decrease or an increase in Tx So ATg is less than 0 if there is a depletion of segments at the boundaries, and AT is greater than 0 if the segment-filler interactions are strong in compari- on with the segment-segment interactions. Furthermore, it EGMRUP was argued by extension that if the system is asymmetric 2 (bounded by a free surface and a substrate), AT is greater than a e monomer-filler interactions are sufficiently strong to dominate the depletion of chain segments near the free surface; otherwise, ATg is less than 0. Moreover, a comparative study carried out of the thermal properties between a functionalized carbon nanotube(CNT)-epoxy composite and a nonfunctional ized CNT-epoxy one revealed a stronger shift in Tg in the case of the composite containing functionalized CNTs. According to this study, it was assumed that covalent bonds between the amino functions on the surface of the CNT and the epoxy Figure 4. Traction curves of the composites NFRUP and EGMRUP Www. MaterialSviewS. cOm WILEYONLINELIBRARY. COM/APP J APPL POLYM. SCL 2013, DOl: 10.1002/APP. 38499 491

that hybrid composites showed significant improvement in fatigue crack propagation resistance, and this could support the NFRUP hybrid composite, which contained two natural fibers (alfa and wool) and the polymeric fibers (PET–PE). A closer look at the E-glass-fiber cross-sectional aspects depicted in Fig￾ure 2(d) demonstrated a similar close contact in the interfacial region between the fibers and the matrix compared with that of the NFRUP composite. DSC The UP resin matrix and its NFRUP and EGMRUP composites were subjected to DSC to evaluate their thermal properties. The thermographs of all of the samples are shown in Figure 3. The Tg value of each sample was determined from the midpoint value of the jump in heat flow after the second heating run. Also, the line construction was done by the Pyris software, and the obtained values were about 68, 78, and 54
C for the resin and the NFRUP and the EGMRUP composites, respectively. These results indicate that Tg, which was attributed to the tran￾sition from a glasslike form to a rubbery and flexible form, increased with the addition of the natural fibers to the matrix. However, this temperature decreased with the addition of E-glass fibers to the matrix. Changes in the glass-transition tem￾perature (DTg) can indicate altered polymer chain mobility. Indeed, depending on the strength of the interaction between the polymer and the filler, this region can have a higher or lower mobility than the bulk material, and this can result in a decrease28 or an increase29 in Tg. 30 So DTg is less than 0 if there is a depletion of segments at the boundaries, and DTg is greater than 0 if the segment–filler interactions are strong in compari￾son with the segment–segment interactions.31–34 Furthermore, it was argued by extension35 that if the system is asymmetric (bounded by a free surface and a substrate), DTg is greater than 0 if the monomer–filler interactions are sufficiently strong to dominate the depletion of chain segments near the free surface; otherwise, DTg is less than 0. Moreover, a comparative study carried out of the thermal properties between a functionalized carbon nanotube (CNT)–epoxy composite and a nonfunctional￾ized CNT–epoxy one revealed a stronger shift in Tg in the case of the composite containing functionalized CNTs. According to this study, it was assumed that covalent bonds between the amino functions on the surface of the CNT and the epoxy would lead to an even stronger reduction in the matrix mobil￾ity, which is expressed by a stronger shift in Tg. 36 The different behaviors of the two sample series provided further evidence of the influence of the chemical functionalization of the surface on the interfacial adhesion between the nanotubes and the epoxy resin. Tensile Properties The interface adhesion between the reinforcing fiber and the matrix markedly influenced the mechanical performance of the composites.37,38 This region is the site synergy in composite materials, as the stress redistribution from the matrix to the fibers takes place through their bond/interphase.39 Therefore, although the interface region appeared to have an insignificant volume fraction, its influence on the overall material properties was significant.40,41 Indeed, it has been observed that the exis￾tence of this interface/interphase brings about significant changes in the relaxation behavior of Tg, as discussed earlier.42,43 Figure 4 illustrates the traction curves of the NFRUP and EGMRUP composites. Both curves showed a plateau at the be￾ginning for a low strength attributed to the slide of the speci￾men in the grips of the dynamometer. After this plateau, the curves exhibited a typical mechanical behavior in each compos￾ite. On the NFRUP composite traction curve, a depression was observed, which could be explained by the stretching of some fibers. Indeed, the tensile properties of the alfa fibers were lower than those of the E-glass fibers,44 and this explained the absence of such a depression on the EGMRUP composite traction curve. Accordingly, Figure 5(a–c) shows the Young’s modulus, strength, and stress at break of the NFRUP and EGMRUP composites. From these histograms, we observed that the tensile properties of the NFRUP composite were slightly better than those of EGMRUP with regard to the obtained values of the Young’s modulus and tensile strength at break values. This showed, according to the Young’s modulus, a good cohesion of the materials to transfer stress from the matrix to the fiber.45 How￾ever, the stress at break of the EGMRUP composite was superior to that of the NFRUP composite; this could be attributed to the greater strength of the E-glass fibers compared to that of the natural fibers, as mentioned previously. When specific proper￾ties were compared, the differences in the tensile performance became more marked. In fact, the NFRUP composite exhibited Figure 3. DSC thermograms of the matrix and its composites (NFRUP and EGMRUP) in the second heating run. Figure 4. Traction curves of the composites NFRUP and EGMRUP. WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2013, DOI: 10.1002/APP.38499 491 ARTICLE

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 LIBRARY

superior 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

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 493

decreased 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

RTICLE Applied Polymer (a) ◆40°C ℃℃℃ 100000 120c 一-130c 10000 100000 1000000 Frequency(Hz) 0 是140°C 1000001000000 F Figure 7. Frequency dependence of M and M for the(a)NFRUP and(b) EGMRUP composites appearance of a relaxation peak associated with the interfacial interpret bulk relaxation properties is that the In olarization for the NFRUP composite. The same result was large values of the real part of the permittivity y and the lo observed by Okrassa et al., in which the a relaxation of the tor at low frequencies are minimized. In this way, common dif- yroxypropyl cellulose related to its glass transition was not visi- ficulties of the electrode nature and contact, space-charge-injo ble in the dielectric spectra. This behavior was connected with tion phenomena, and absorbed impurity conduction effects, he presence of strong hydrogen bonds between cellulose chains. which appear to hide the relaxation in the permittivity repre- As already shown, the dc conductivity can hide the a relaxation sentation, can be solved or even ignored.59 in dielectric spectra, so to minimize this effect, the formalism of With this M formalism adopted, Figure 7(a, b) shows the simi- the electric modulus(M) or inverse &* was introduced. This lar behavior of M as a function of frequency for the NFRUP M has recently been adapted for the investigation of dielectric and EGMRUP composites when they were heated over the tem- processes occurring in composite polymeric systems and those perature range from 40 to 150%C. The inset of each figure proposed for the description of systems with ionic conductiv- exhibits the isothermal variation of the M frequency depend ity. M is defined by Eq (4): ence. The value of M was nearly zero at low frequencies and high temperatures; this indicated that the electrode polarization M*== E+e222+e2+M+M"(4) gave a negligibly low contribution to M and could be g ignored.0,b After this initial low value, M increased steeply in the range of 10. A series of two distinct relaxations could be where M and M are the real and imaginary parts of the considered for each composite. The first one was related to the electric modulus, respectively. An advantage of using M to a relaxation associated with the glass-rubbery transition of the 494 J. APPL. POLYM.Sc.2013,D0:10.1002PP38499 WILEYONLINELIBRARY. COM/APP EWILEY NONLINE LIBRARY

appearance of a relaxation peak associated with the interfacial polarization for the NFRUP composite. The same result was observed by Okrassa et al.,55 in which the a relaxation of the hyroxypropyl cellulose related to its glass transition was not visi￾ble in the dielectric spectra. This behavior was connected with the presence of strong hydrogen bonds between cellulose chains. As already shown, the dc conductivity can hide the a relaxation in dielectric spectra, so to minimize this effect, the formalism of the electric modulus (M*) or inverse e* was introduced. This M* has recently been adapted for the investigation of dielectric processes occurring in composite polymeric systems and those proposed for the description of systems with ionic conductiv￾ity.12 M* is defined by Eq. (4):58 M ¼ 1 e ¼ 1 e0 je00 ¼ e0 e02 þ e002 þ j e00 e02 þ e002 þ M0 þ jM00 (4) where M0 and M00 are the real and imaginary parts of the electric modulus, respectively. An advantage of using M* to interpret bulk relaxation properties is that the variation in the large values of the real part of the permittivity and the loss fac￾tor at low frequencies are minimized. In this way, common dif￾ficulties of the electrode nature and contact, space-charge-injec￾tion phenomena, and absorbed impurity conduction effects, which appear to hide the relaxation in the permittivity repre￾sentation, can be solved or even ignored.59 With this M* formalism adopted, Figure 7(a,b) shows the simi￾lar behavior of M00 as a function of frequency for the NFRUP and EGMRUP composites when they were heated over the tem￾perature range from 40 to 150
C. The inset of each figure exhibits the isothermal variation of the M0 frequency depend￾ence. The value of M0 was nearly zero at low frequencies and high temperatures; this indicated that the electrode polarization gave a negligibly low contribution to M0 and could be ignored.60,61 After this initial low value, M0 increased steeply in the range of 10. A series of two distinct relaxations could be considered for each composite. The first one was related to the a relaxation associated with the glass–rubbery transition of the Figure 7. Frequency dependence of M0 and M00 for the (a) NFRUP and (b) EGMRUP composites. 494 J. APPL. POLYM. SCI. 2013, DOI: 10.1002/APP.38499 WILEYONLINELIBRARY.COM/APP ARTICLE

Applied Polymer ARTICLE EGMRUP 150.2025 Figure 8. Arrhenius plots of the t values versus the reciprocal tempera- ture for the NFRUP and EGMRUP composites polymer, and the second one, which appeared at high tempera tures, was attributed to an ionic conduction effect. to further support these assignments, the activation energy (Ea) relative to the different relaxations was evaluated with the following Arrhe- nius relation: where t(= 1/2rfmax)is the relaxation time associated with the maximum M for a fixed temperature, to is the relaxation time 0.10.1502 at very high temperatures, Ea is the activation energy of the relaxation process, kg is the Boltzmann constant, and T is the Figure 9. Argand's plots of M of the (a) NFRUP and(b)Egmrup temperature. Figure 8 shows the evolution of log t versus 1/T composites at 150C for each one of the different observed relaxations: that is, x and conductivity for the NFRUP and EGMRUP composites. Ea and ities for the NFRUP composite (96.33 k]/mol for a and to were extracted from the slopes and the intercepts of the plots 10-15.47 s for to) and the EGMRUP composite(100 k]/mol for of log t versus 1/T. The mean values of Ea and to relative to the Ea and 10-15. s for to). These values were in agreement with a relaxation were 136.02 kJ/mol and 10 s and 123. 2 kJ/mol those reported in other research works. 62 and 10 s for the NFRUP and EGMRUP composites, respec- tively, as mentioned in Table Il. However, it can be noted that The Argand representation was used to analyze the nature of the incorporation of fibers decreased the apparent activation the relaxation. Cole-Cole plots of the NFRUP and EGMRUP the matrix determined in the previous study; this could be well established that the response of every relaxation mechanism This interaction determined the nature of the interfacial adhe- parameters at the most. Among others, this includes the fol sion region. The latter was characterized with SEM, as illus- trated in the previous section. It also determined the conducti Table IL. Activation Energies Ea and Relaxation Times to for the NFRUP 1+(ior)] and EGMRUP Composite Materials. This function was introduced by Havriliak-Negami and is widely used because of its suitability for mathematical process- Composite material E。化kJm 6 In this equation, Es and Eoo are the dielectric constants on NFRUP the low-and high-frequency sides of the relaxation, t is 13602 tral relaxation time, o is the radial frequency, and a and Bare Conduction 9633 0-15.47 fractional shape parameters describing the skewing and broad EGMRUP ening of the dielectric function, respectively. Both a and B range a Relaxation 1232 between 0 and 1. These coefficients act as the deviation from the Debye equation. In fact, when a and B are equal to 1, this Conduction 100 10-1586 equation reduces to the Debye equation. In the M formalism Www. MaterialSviewS. cOm WILEYONLINELIBRARY. COM/APP J APPL POLYM. SCL 2013, DOl: 10.1002/APP. 38499 495

polymer, and the second one, which appeared at high tempera￾tures, was attributed to an ionic conduction effect. To further support these assignments, the activation energy (Ea) relative to the different relaxations was evaluated with the following Arrhe￾nius relation: s ¼ s0 exp Ea kBT
(5) where s (¼ 1/2pfmax) is the relaxation time associated with the maximum M00 for a fixed temperature, s0 is the relaxation time at very high temperatures, Ea is the activation energy of the relaxation process, kB is the Boltzmann constant, and T is the temperature. Figure 8 shows the evolution of log s versus 1/T for each one of the different observed relaxations; that is, a and conductivity for the NFRUP and EGMRUP composites. Ea and s0 were extracted from the slopes and the intercepts of the plots of log s versus 1/T. The mean values of Ea and s0 relative to the a relaxation were 136.02 kJ/mol and 1020.96 s and 123.2 kJ/mol and 1019 s for the NFRUP and EGMRUP composites, respec￾tively, as mentioned in Table II. However, it can be noted that the incorporation of fibers decreased the apparent activation energy (Ea a) for the two composites in comparison with that for the matrix determined in the previous study; this could be ascribed to the interaction between the fibers and the matrix.20 This interaction determined the nature of the interfacial adhe￾sion region. The latter was characterized with SEM, as illus￾trated in the previous section. It also determined the conductiv￾ities for the NFRUP composite (96.33 kJ/mol for Ea and 1015.47 s for s0) and the EGMRUP composite (100 kJ/mol for Ea and 1015.86 s for s0). These values were in agreement with those reported in other research works.62 The Argand representation was used to analyze the nature of the relaxation. Cole–Cole plots of the NFRUP and EGMRUP composites at 150
C are depicted in Figure 9(a,b). It has been well established that the response of every relaxation mechanism can be represented very precisely by a model function with four parameters at the most. Among others,63 this includes the fol￾lowing function: e ¼ e1 þ eS e1 ½1 þ ðixsÞ a  b (6) This function was introduced by Havriliak–Negami and is widely used because of its suitability for mathematical process￾ing.64 In this equation, eS and e1 are the dielectric constants on the low- and high-frequency sides of the relaxation, s is the cen￾tral relaxation time, x is the radial frequency, and a and b are fractional shape parameters describing the skewing and broad￾ening of the dielectric function, respectively. Both a and b range between 0 and 1. These coefficients act as the deviation from the Debye equation. In fact, when a and b are equal to 1, this equation reduces to the Debye equation. In the M* formalism, Figure 8. Arrhenius plots of the s values versus the reciprocal tempera￾ture for the NFRUP and EGMRUP composites. Table II. Activation Energies Ea and Relaxation Times s0 for the NFRUP and EGMRUP Composite Materials. Composite material Ea (kJ/mol) s0 (s) NFRUP a Relaxation 136.02 1020.96 Conduction 96.33 1015.47 EGMRUP a Relaxation 123.2 1019 Conduction 100 1015.86 Figure 9. Argand’s plots of M* of the (a) NFRUP and (b) EGMRUP composites at 150
C. WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2013, DOI: 10.1002/APP.38499 495 ARTICLE

ARTICLE Applied Polymer Table Ill. Parameters Evaluated by Data Fitting According to the Havriliak-Negami Equation for the NFRUP and EGMRUP Composites Composite material T(C) Relaxation NFRUP 120 Conduction 0.991 0.947 0003 026 MWS 0.892 0.199 0.315 130 Conduction 0.99150.90 0 .295 0.901 0.88 031 140 Conduction 0.995 0002 0298 0.999 0.21 0316 150 Conduction 0.999 0.904 0.0 0033 0301 MWS 0.99 0863 0.153 03182 EGMRUP 120 Conduction 0.9957 0.889 00052 0.31 Conduction 005 140 0.999 0.819 000399 0.3269 the Havriliak-Negami equations [eqs. (7)and (8)) have the fol- values of a, B, Ms, and Mo that best smoothed the Havriliak- lowing form: Negami data were obtained by a successive approach method, in which the following expressions were minimized M=M MsA+(Moc-MS)cos BoJA/ MAZF(Moo-M,)M, cos Bo+(M-M, )2 r=∑(M-Mp)2 M=MM (M、-M) sin BoJA MEA2(Ms-Ms)M, cos Bo +(M-M(8) It has been proven that only one quadruplet value was able to tone with these conditions. Although the values of a and B obtained for the conductive effect were in harmony with a pu Debye type, the values of a and B obtained for interfacial pola (9) ton were in accordance with the Havriliak-Negami respons CONCLUSIONS Moc= (10) A comparative study between natural-fiber-matrix(NFRUP) and E-glass-matrix (EGMRUP) composites for thermal and A=1+2(o)-2sin+(ox)2(-y12 (11) dielectric properties was undertaken. The UP resin was used as a matrix for both of them. The thermal study(DSC)carried (ot)cos &3 (12) out on these samples showed a variation in Tg as fibers(natural 1+(or)si or mineral) were added in the matrix, and this revealed the interaction of the fibers with the matrix. The dielectric response Accordingly, dotted curves were produced by the best fitting ex- of these composites showed the presence of two common perimental points with the Havriliak-Negami equations [eqs. dielectric relaxations, which were attributed to the a relaxation (7)and(S). In Figure 9(b), the Cole-Cole diagram corre- of the polymer and to ionic conduction, which occurred above sponded to the conductivity effect for the EGMRUP composite, the glass transition and at low frequencies, respectively. The whereas in Figure 8(a), it is shown that it was impossible to fit dielectric analysis revealed that interfacial polarization could not the Havriliak-Negami model to all of the experimental points for be analyzed by the Argand representation in the EGMRUP com- the NFRUP composites So, two semicircles were obtained at ev- posite, as it was absent, whereas in the case of the NFRUP com- ery examined temperature. The first one for 0< M<0.3 was posite, the analysis of the MwS polarization exhibited a consis related to the conduction effect, and the second one for 0. 15< tency of this polarization with the Havriliak-Negami model. M<0.32 was linked to the MWS effect. This analysis confirmed The tensile properties of these composites evidenced a slight the presence of the MWS relaxation, which is overlapped with enhancement in the fiber-matrix adhesion in favor of the he dc conductivity effect. The parameters evaluated by the fitting NFRUP composite. As a parameter closely related to the static stress transfer at the interface, the Youngs modulus showed a teristics of the Havriliak-Negami model (a, B, Ms, and Moc), the slight enhancement in the NFRUP composite compared with experimental Mexp and Mex data were smoothed through a nu- that in the EGMRUP composite Moreover, this comparative ex merical simulation in the complex plane. The purpose of such a perimental study demonstrated that natural fibers enhanced the simulation was to find the theoretical values(M, and M, h ). The thermal insulation in the NFRUP composite according to the 496 J. APPL. POLYM.Sc.2013,D0:10.1002PP38499 WILEYONLINELIBRARY. COM/APP EWILEY NONLINE LIBRARY

the Havriliak–Negami equations [eqs. (7) and (8)] have the fol￾lowing form:64 M0 ¼ M1 ½MsAb þ ðM1 MsÞ cos buAb M2 SA2bðM1 MsÞMs cos bu þ ðM1 MsÞ 2 (7) M00 ¼ M1Ms ½ðM1 MsÞsin buAb M2 s A2bðM1 MsÞMs cos bu þ ðM1 MsÞ 2 (8) where Ms ¼ 1 es (9) M1 ¼ 1 e1 (10) A ¼ ½1 þ 2ðxsÞ 1a sin ap 2 þ ðxsÞ 2ð1aÞ  1=2 (11) u ¼ arctg½ ðxsÞ 2a cose ap 2 1 þ ðxsÞ 1a sin ap 2  (12) Accordingly, dotted curves were produced by the best fitting ex￾perimental points with the Havriliak–Negami equations [eqs. (7) and (8)]. In Figure 9(b), the Cole–Cole diagram corre￾sponded to the conductivity effect for the EGMRUP composite, whereas in Figure 8(a), it is shown that it was impossible to fit the Havriliak–Negami model to all of the experimental points for the NFRUP composites. So, two semicircles were obtained at ev￾ery examined temperature. The first one for 0 < M0 < 0.3 was related to the conduction effect, and the second one for 0.15 < M0 < 0.32 was linked to the MWS effect. This analysis confirmed the presence of the MWS relaxation, which is overlapped with the dc conductivity effect. The parameters evaluated by the fitting data are listed in Table III. To determine the parameters charac￾teristics of the Havriliak–Negami model (a, b, MS, and M1), the experimental M0 exp and M00 exp data were smoothed through a nu￾merical simulation in the complex plane. The purpose of such a simulation was to find the theoretical values (M0 th and M00 th). The values of a, b, MS, and M1 that best smoothed the Havriliak– Negami data were obtained by a successive approach method, in which the following expressions were minimized: v2 M0 ¼ X i ðM0 th M0 expÞ 2 (13) v2 M00 ¼ X i ðM00 th M00 expÞ 2 (14) It has been proven that only one quadruplet value was able to tone with these conditions. Although the values of a and b obtained for the conductive effect were in harmony with a pure Debye type, the values of a and b obtained for interfacial polar￾ization were in accordance with the Havriliak–Negami response. CONCLUSIONS A comparative study between natural-fiber–matrix (NFRUP) and E-glass–matrix (EGMRUP) composites for thermal and dielectric properties was undertaken. The UP resin was used as a matrix for both of them. The thermal study (DSC) carried out on these samples showed a variation in Tg as fibers (natural or mineral) were added in the matrix, and this revealed the interaction of the fibers with the matrix. The dielectric response of these composites showed the presence of two common dielectric relaxations, which were attributed to the a relaxation of the polymer and to ionic conduction, which occurred above the glass transition and at low frequencies, respectively. The dielectric analysis revealed that interfacial polarization could not be analyzed by the Argand representation in the EGMRUP com￾posite, as it was absent, whereas in the case of the NFRUP com￾posite, the analysis of the MWS polarization exhibited a consis￾tency of this polarization with the Havriliak–Negami model. The tensile properties of these composites evidenced a slight enhancement in the fiber–matrix adhesion in favor of the NFRUP composite. As a parameter closely related to the static stress transfer at the interface, the Young’s modulus showed a slight enhancement in the NFRUP composite compared with that in the EGMRUP composite. Moreover, this comparative ex￾perimental study demonstrated that natural fibers enhanced the thermal insulation in the NFRUP composite according to the Table III. Parameters Evaluated by Data Fitting According to the Havriliak–Negami Equation for the NFRUP and EGMRUP Composites Composite material T (
C) Relaxation a b MS M1 NFRUP 120 Conduction 0.991 0.947 0.003 0.26 MWS 0.92 0.892 0.199 0.315 130 Conduction 0.9915 0.90 0.002 0.295 MWS 0.901 0.88 0.185 0.3191 140 Conduction 0.995 0.93 0.002 0.298 MWS 0.999 0.92 0.21 0.316 150 Conduction 0.999 0.904 0.0033 0.301 MWS 0.99 0.863 0.153 0.3182 EGMRUP 120 Conduction 0.9957 0.889 0.0052 0.31 130 Conduction 0.995 0.83 0.005 0.32 140 Conduction 0.999 0.819 0.00399 0.3269 496 J. APPL. POLYM. SCI. 2013, DOI: 10.1002/APP.38499 WILEYONLINELIBRARY.COM/APP ARTICLE