《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