polymers

Article Prunifera Fiber: A Promising New Reinforcement for Epoxy Composites

Raí Felipe Pereira Junio 1, Lucio Fabio Cassiano Nascimento 1 , Lucas de Mendonça Neuba 1 , Andressa Teixeira Souza 1 , João Victor Barbosa Moura 2 ,Fábio da Costa Garcia Filho 3 and Sergio Neves Monteiro 1,*

1 Department of Materials Science, Military Institute of Engineering-IME, Rio de Janeiro 22290270, ; [email protected] (R.F.P.J.); [email protected] (L.F.C.N.); [email protected] (L.d.M.N.); [email protected] (A.T.S.) 2 Science and Technology Center, Federal University of Cariri, Juazeiro do Norte 63048080, Brazil; [email protected] 3 Department of Mechanical and Aerospace Engineering, University of California San Diego—UCSD, La Jolla, CA 92093, USA; fdacostagarciafi[email protected] * Correspondence: [email protected] or [email protected]

 Received: 17 August 2020; Accepted: 11 September 2020; Published: 14 September 2020 

Abstract: A basic characterization of novel epoxy matrix composites incorporated with up to 40 vol% of processed leaf fibers from the palm , known as carnauba fibers, was performed. The tensile properties for the composite reinforced with 40 vol% of carnauba fibers showed an increase (40%) in the tensile strength and (69%) for the elastic modulus. All composites presented superior elongation values in comparison to neat epoxy. Izod impact tests complemented by fibers/matrix interfacial strength evaluation by pullout test and Fourier transformed infrared (FTIR) analysis revealed for the first time a significant reinforcement effect (> 9 times) caused by the carnauba fiber to polymer matrix. Additional thermogravimetric analysis (TG/DTG) showed the onset of thermal degradation for the composites (326 ~ 306 ◦C), which represents a better thermal stability than the plain carnauba fiber (267 ◦C) but slightly lower than that of the neat epoxy (342 ◦C). Differential scanning calorimetry (DSC) disclosed an endothermic peak at 63 ◦C for the neat epoxy associated with the glass transition temperature (Tg). DSC endothermic peaks for the composites, between 73 to 103 ◦C, and for the plain carnauba fibers, 107 ◦C, are attributed to moisture release. Dynamic mechanical analysis confirms Tg of 64 ◦C for the neat epoxy and slightly higher composite values (82–84 ◦C) due to the carnauba fiber interference with the epoxy macromolecular chain mobility. Both by its higher impact resistance and thermal behavior, the novel carnauba fibers epoxy composites might be considered a viable substitute for commonly used glass fiber composites.

Keywords: Copernicia prunifera; carnauba fibers; epoxy matrix; natural fibers composites; characterization

1. Introduction Sustainable action to mitigate worldwide pollution and climate changes are promoting the use of natural materials in the substitution for synthetic ones. A typical example is the use of fibers extracted from replacing glass fibers as reinforcement in polymer matrix composites [1–3]. Indeed, composites reinforcement with natural lignocellulosic fibers (NLFs) are likely to be environmentally friendly than glass fiber composites (fiberglass) in terms of biodegradability and reduced process energy [1]. Moreover, the density-rationalized specific strength of some NLF composites are superior to that of fiberglass [2]. In addition, NLFs are comparatively less expensive and nonabrasive to processing

Polymers 2020, 12, 2090; doi:10.3390/polym12092090 www.mdpi.com/journal/polymers Polymers 2020, 12, 2090 2 of 20

Polymers 2020, 12, x 2 of 20 equipments, which contribute to their cost effectiveness [4]. Another relevant advantage is the social andbenefit nonabrasive since, around to processing the world, equipments, many NLFs arewhich cultivated contribute in developing to their cost regions effectiveness and represent [4]. Another a major relevantsource of advantage income to theis the local social population benefit [ 5since,]. around the world, many NLFs are cultivated in developingA surging regions in the and interest represent for polymer a major compositessource of income incorporated to the local with population natural fibers [5]. in past decades motivatedA surging a significant in the interest number for of polymer research composites works [6–15 incorporated] and industrial with application natural fibers [16–20 in]. past In particular, decades motivatedless known a NLFs,significant such asnumber guaruman of research [21], buriti works [22], [6–15] and fique and [ 23industrial] have recently application been successfully[16–20]. In particular,evaluated asless reinforcement known NLFs, of such polymer as guaruman composites. [21] Another, buriti less[22], known and fique NLF [2 is3] the have fiber recently extract been from successfullythe leaf-stalk evaluated of the Copernicia as reinforc pruniferaement ofpalm polymer tree, endemiccomposites. in theAnother northeastern less known of Brazil NLF is where the fiber it is extractreferred from to as the canaubeira. leaf-stalk Figureof the 1Coperniciaa illustrates prunifera a plantation palm tree, of carnaubeiras, endemic in andthe northeastern Figure1b shows of Brazil their wheretypical it leavesis referred composed to as canaubeira. of radial stalks. Figure The1a illu mainstrates international a plantation valuable of carnaubeiras, product ofand carnaubeira Figure 1b showsCopernicia their prunifera typical leavesis the carnaubacomposed wax of radial worldwide stalks. commercialized The main internatio in thenal form valuable of yellow-brown product of carnaubeiraflakes [24]. ThisCopernicia wax has prunifera multiple is applications.the carnauba It wax can produceworldwide a glossy commercialized finish in automobile, in the form shoes, of yellow-brownfurniture, surfboard, flakes and[24]. floor.This Inwax addition has multiple to gloss, applications. the hypoallergenic It can produce and emollient a glossy properties finish in of automobile, shoes, justify furniture, its use insurfboard, cosmetics, and skin floor. care, In and addition even candy to gloss, coating. the hypoallergenic In the US, the mainand emollientimporter properties of carnauba of wax, carnauba its most wax common justify its application use in cosmetics, is for paper skin care, coatings and [even25]. Aftercandy removing coating. Inthe the , US, the cellulose-rich main importer stalks of carnauba are considered wax, its most waste common and usually application disposed is for on paper the coatings soil or burnt. [25]. AfterSustainable removing destination the leaves, for cellulose-rich this increasing stalks amount are considered of waste waste could and be achievedusually disposed by considering on the soil its orincorporation burnt. Sustainable in polymer destination composites. for this increasing amount of waste could be achieved by considering its incorporation in polymer composites.

FigureFigure 1. 1. PlantationPlantation of of carnaubeiras carnaubeiras ( (aa)) and and typical typical leaf-stalks leaf-stalks of of carnauba carnauba tree tree ( (bb).).

To our knowledge, a single research work on carnauba fiber incorporated polymer composites To our knowledge, a single research work on carnauba fiber incorporated polymer composites has been published so far. Melo et al. [26] investigated the 10 wt% incorporation of chemically has been published so far. Melo et al. [26] investigated the 10 wt% incorporation of chemically modified short-cut carnauba fibers into biodegradable polyhydroxybutyrate (PHB) matrix composites. modified short-cut carnauba fibers into biodegradable polyhydroxybutyrate (PHB) matrix Mechanical properties of their untreated fibers, together with corresponding results of related composites. Mechanical properties of their untreated fibers, together with corresponding results of composites, are presented in Table1. related composites, are presented in Table 1.

Table 1. Mechanical properties of plain untreated carnauba fiber neat polyhydroxybutyrate (PHB) and Table 1. Mechanical properties of plain untreated carnauba fiber neat polyhydroxybutyrate (PHB) 10 wt% carnauba fiber PHB composites, reproduced from [26]. and 10 wt% carnauba fiber PHB composites, reproduced from [26].

Material Tensile StrengthTensile (MPa) Strength Young’s Young’s Modulus Modulus (GPa) Material Neat PHB 28–30(MPa) 3.3–3.6(GPa) Plain untreated carnauba fiber 205–264 8.2–9.2 Neat PHB 28–30 3.3–3.6 10 wt% untreated with carnauba fiber/PHB 17–19 2.1–2.7 Plaincomposite untreated carnauba fiber 205–264 8.2–9.2 10Maximum wt% untreated values with for a 10carnauba wt% chemically fiber/PHB composite 17–19 2.1–2.7 24–27 3.0–3.4 Maximummodified carnaubavalues for fiber a 10/PHB wt% composite chemically modified 24–27 3.0–3.4 carnauba fiber/PHB composite

An important point to be noted in Table 1 is the relatively high tensile strength and Young’s modulus of the plain untreated carnauba fiber. It is, however, surprising that 10 wt% addition of this fiber, either untreated or chemically modified, into the PHB matrix was not able to promote any

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An important point to be noted in Table1 is the relatively high tensile strength and Young’s Polymersmodulus 2020 of, 12 the, x plain untreated carnauba fiber. It is, however, surprising that 10 wt% addition3 of 20 of this fiber, either untreated or chemically modified, into the PHB matrix was not able to promote reinforcement.any reinforcement. As such, As such, the theonly only published published results results on on carnauba carnauba fiber fiber incorporated incorporated polymer polymer compositescomposites [26], [26], failed failed to to show a reinforcement eeffffect.ect. Based Based on on the the relatively relatively high high mechanical mechanical propertiesproperties of carnauba fiberfiber inin Table Table1, 1, which which are are comparable comparable to otherto other NLFs, NLFs, such such as bamboo, as bamboo, coir, coir, and andpiassava piassava [4] used [4] asused reinforcement as reinforcement for polymer for polymer composites, composites, an investigation an investigation deserves todeserves be conducted to be conductedon this less on known this less fiber. known fiber. Therefore,Therefore, in in the the present present work work the the possible possible us usee of of carnauba carnauba fiber fiber as as reinforcement reinforcement of of epoxy epoxy compositescomposites is is for for the the first first time time investigated investigated regarding regarding the the tensile tensile properties, properties, impact impact resistance resistance by by Izod Izod tests,tests, as as well well as as fiber/matrix fiber/matrix interfacial strength obtained by pullout tests.tests. The The Fourier Fourier transform transform infraredinfrared (FTIR)(FTIR) spectroscopy spectroscopy and and thermal thermal analysis analys contributeis contribute to characterize to characterize the limits the for engineeringlimits for engineeringapplications applications of these composites, of these which composites, are expected which to are be establishedexpected to in be the established future ongoing in the works future by ongoingour research works group. by our research group.

2.2. Materials Materials and and Methods Methods

2.1.2.1. Materials Materials CarnaubaCarnauba greengreen leavesleaves with with stalks, stalks, illustrated illustrate ind Figure in Figure2a, were 2a, purchasedwere purchased from a ruralfrom producer a rural producerin the city in of the Barro, city stateof Barro, of Cear stateá northeast of regionnortheast of Brazil. region The of Brazil. as-received The as-received leaves were leaves subjected were to subjectedimmersion to in immersion water for 24in h,water followed for 24 by h, sun-dryingfollowed by in sun-drying open air for in 12 open h and air then for cleaning12 h and before then cleaningfiber-shredding before fiber-shredding for the final aspect for showthe final in Figure aspect2 b.show Preliminary in Figure measurements 2b. Preliminary of measurements the fibers density of indicate an average value of 1.13 0.21 g/cm3. the fibers density indicate an average± value of 1.13 ± 0.21 g/cm3.

Figure 2. Macroscopic aspect of carnauba: (a) As received leaf-stalks and (b) shredded fibers. Figure 2. Macroscopic aspect of carnauba: (a) As received leaf-stalks and (b) shredded fibers. The material used as matrix for the composite plates was a commercial epoxy resin type ether, diglycidylThe material bisphenol used A as (DGEBA), matrix for hardenedthe composite with pl triethyleneates was a commercial tetramine (TETA), epoxy resin associated type ether, with diglycidyla stoichiometric bisphenol ratio A of (DGEBA), 13 parts of hardened hardener towith 100 tr partsiethylene of resin. tetramine The resin (TETA), manufacturer associated was with Dow a stoichiometricChemical (São ratio Paulo), of supplied13 parts of and hardener distributed to 100 by parts Epoxy of Fiber resin. (Rio The de resin Janeiro), manufacturer both in Brazil. was Dow Chemical (São Paulo), supplied and distributed by Epoxy Fiber (Rio de Janeiro), both in Brazil. 2.2. Composites Processing 2.2. Composites Processing The fibers shown in Figure2b were once again cleaned, cut to 150 mm in length and finally, dried in anThe oven fibers at 30 shown◦C for 24in h,Figure to release 2b were excess once of humidity,again cleaned, as shown cut to in 150 Figure mm3b. in For length the manufacture and finally, driedof composite in an oven plates, at a30 steel °C mold,for 24 Figureh, to 3releasea, with excess an internal of humidity, volume of as 15 shown12 in1.19 Figure cm3 was3b. adaptedFor the × × manufactureto a hydraulic of press composite (Skay, Splates,ão Jos éa dosteel Rio mold, Preto, Figure SP, Brazil) 3a, with with an maximum internal volume load capacity of 15 × of 12 30 × tons. 1.19 cm3 was adapted to a hydraulic press (Skay, São José do Rio Preto, SP, Brazil) with maximum load capacity of 30 tons.

Polymers 2020, 12, 2090 4 of 20 Polymers 2020, 12, x 4 of 20

FigureFigure 3. 3.Methodology Methodology forfor processingprocessing the composite composite plates: plates: (a ()a Metallic) Metallic mold, mold, (b) ( bcleaned) cleaned and and dried dried carnaubacarnauba fibers, fibers, and and ( c(c)) curedcured compositecomposite plate.

InIn order order to to fabricate fabricate thethe compositecomposite plate, plate, contin continuousuous and and aligned aligned carnauba carnauba fibers fibers were were precisely precisely handhand lay-up lay-up along along the the mold’s mold’s 15 cm 15 greater cm greater dimension. dimension. The amount The amount of fiber of for fiber each platefor each corresponds plate tocorresponds its defined volumeto its defined fraction volume in the fraction composite. in the Still composite. fluid epoxy Still fluid resin epoxy DGEBA-TETA resin DGEBA-TETA was carefully pouredwas carefully into the poured mold avoidinginto the mold fibers avoiding movement. fibers Themovement. system The was system then sealedwas then and sealed subjected and to a pressuresubjected ofto 5a tonspressure for 24 of h. 5 Figuretons for3 c24 shows h. Figure a typical 3c shows cured a compositetypical cured plate composite in which plate most in fiberswhich are wellmost aligned, fibers are which well confirms aligned, thatwhich fibers confirms did not that move fibers during did not the move 24 h during pressing. the For24 h the pressing. calculation For of thethe volume calculation fraction of the of thevolume composite, fraction the of densitythe composite, of 1.11 gthe/cm density3 was usedof 1.11 for g/cm the DGEBA-TETA3 was used for the epoxy 3 resinDGEBA-TETA [27] and 1.34 epoxy g/cm resin3 for [27] the carnaubaand 1.34 g/cm fibers for [26 the]. Compositecarnauba fibers plates [26] with. Composite 0, 10, 20, plates 30, and with 40 vol%0, of10, fibers 20, 30, were and prepared. 40 vol% of fibers were prepared.

2.3.2.3. Pullout Pullout Test Test FigureFigure4 schematically4 schematically illustrates illustrates thethe specimen used used for for the the pullout pullout test, test, which which is iscomposed composed of of cylindrical blocks with 8 mm in diameter. These specimens were prepared by varying the depth of cylindrical blocks with 8 mm in diameter. These specimens were prepared by varying the depth of single fiber inlay embedded length of 1.5 to 30 mm, according to the methodology proposed by Kelly single fiber inlay embedded length of 1.5 to 30 mm, according to the methodology proposed by Kelly and Tysson [28] and adapted by Monteiro and D’Almeida [29] for NFLs. The tests were conducted at and Tysson [28] and adapted by Monteiro and D’Almeida [29] for NFLs. The tests were conducted at room temperature (~ 25 °C) in a model 3365 Instron universal machine (Instron Corp., Norwood, MA, room temperature (~ 25 C) in a model 3365 Instron universal machine (Instron Corp., Norwood, MA, USA), with a crosshead ◦speed of 0.75 mm/min. This is a methodology which has been widely used to USA),determine with athe crosshead shear strength speed related of 0.75 to mm the/min. fiber/matr This isixa interface, methodology in particular which hasfor natural been widely fibers used[30– to determine32]. After thetests, shear specimens strength were related gold to sputtered the fiber /inmatrix a vacuum interface, desk inV (Denton, particular TX, for USA) natural and fibers analyzed [30– 32]. Afterby scanning tests, specimens electron microscopy were gold sputtered(SEM) inn ina model a vacuum Quanta desk FEG V (Denton,250 FEI microscope TX, USA) (Field and analyzed Electron by scanningand Ion electronCo., Hillsboro, microscopy OR, USA). (SEM) inn a model Quanta FEG 250 FEI microscope (Field Electron and Ion Co., Hillsboro, OR, USA).

2.4. Tensile Test For the tensile tests, four specimens of each group were cut from the composite plates, following the dimensions required by the ASTM D3039 standard [33], 150 15 2 mm, and gauge length of × × 90 mm. The tests were conducted at room temperature (~25 ◦C) in the same Instron universal machine, with a load capacity of 20 kN and a crosshead speed of 2 mm/min.

Figure 4. Scheme for pullout test (a) and (b) epoxy-carnauba specimen.

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Figure 3. Methodology for processing the composite plates: (a) Metallic mold, (b) cleaned and dried carnauba fibers, and (c) cured composite plate.

In order to fabricate the composite plate, continuous and aligned carnauba fibers were precisely hand lay-up along the mold’s 15 cm greater dimension. The amount of fiber for each plate corresponds to its defined volume fraction in the composite. Still fluid epoxy resin DGEBA-TETA was carefully poured into the mold avoiding fibers movement. The system was then sealed and subjected to a pressure of 5 tons for 24 h. Figure 3c shows a typical cured composite plate in which most fibers are well aligned, which confirms that fibers did not move during the 24 h pressing. For the calculation of the volume fraction of the composite, the density of 1.11 g/cm3 was used for the DGEBA-TETA epoxy resin [27] and 1.34 g/cm3 for the carnauba fibers [26]. Composite plates with 0, 10, 20, 30, and 40 vol% of fibers were prepared.

2.3. Pullout Test Figure 4 schematically illustrates the specimen used for the pullout test, which is composed of cylindrical blocks with 8 mm in diameter. These specimens were prepared by varying the depth of single fiber inlay embedded length of 1.5 to 30 mm, according to the methodology proposed by Kelly and Tysson [28] and adapted by Monteiro and D’Almeida [29] for NFLs. The tests were conducted at room temperature (~ 25 °C) in a model 3365 Instron universal machine (Instron Corp., Norwood, MA, USA), with a crosshead speed of 0.75 mm/min. This is a methodology which has been widely used to determine the shear strength related to the fiber/matrix interface, in particular for natural fibers [30– 32]. After tests, specimens were gold sputtered in a vacuum desk V (Denton, TX, USA) and analyzed byPolymers scanning2020, 12electron, 2090 microscopy (SEM) inn a model Quanta FEG 250 FEI microscope (Field Electron5 of 20 and Ion Co., Hillsboro, OR, USA).

FigureFigure 4. Scheme 4. Scheme for pullout for pullout test test(a) and (a,b )(b epoxy-carnauba) epoxy-carnauba specimen. specimen.

2.5. Izod Impact Test Izod impact tests were carried out according to the ASTM D256-10 standard [34]. Notched prismatic specimens were machined in the direction parallel to the fiber alignment and tested using a Philpolymer instrumented pendulum model XJC 25D (Philpolymer, São Roque, SP, Brazil), using the 22 J hammer. The specimens were produced in the dimensions of 62.5 12.7 10 mm with × × a 2.54 mm notch transversal to the fiber alignment. A minimum of 16 specimens were tested for each volumetric fraction of carnauba fiber investigated. The analysis of variance (ANOVA) was applied, using the F test, in order to assess whether there was a significant difference between the results obtained for Izod impact energy. The 95% confidence level was adopted for the tests, where the alternative hypothesis (H1) is assumed if the value of F (calculated) is higher than the critical Fc (tabulated), thus concluding that at the significance level of 5% there is a difference between the averages of the treatments applied. Given this information, the use of the Tukey test, known as the honestly significant difference test (HSD), becomes necessary. The objective is to quantitatively assess each of the percentages of fibers used. The use of the Tukey test allows the comparison between the averages obtained, two by two, for each of the treatments used (percentage of fibers). From the results, it is possible to reject or not the hypothesis of equality between the averages compared through the lower significant difference (LSD), according to: r MSE LSD = q (1) × r By using this methodology, it was possible to quantitatively determine in a comparative way the influence of the volumetric fraction of the carnauba fibers applied in the production of the composites. The fracture surfaces of the Izod specimens together with the pullout specimens were analyzed by scanning electron microscopy (SEM) in a Quanta FEG 250 FEI microscope, operated with secondary electrons at 10 KV.

2.6. FTIR Analysis The Fourier transform infrared spectroscopy (FTIR) analysis was performed in a Spectrum Two Perkin Elmer (Waltham, MA, USA) equipment. The fibers and composites were ground in the required 1 powder condition to produce the sample tablets. The samples were scanned from 4000 to 450 cm− Polymers 2020, 12, 2090 6 of 20 and the data generated were treated using the equipment data analysis program. The respective 1 transmittance (%) spectra were generated as a function of the wave number (cm− ).

2.7. Thermal Analysis (TG/DTG) For thermogravimetric analysis, both composites and fibers were crushed and allocated in a platinum crucible introduced in a Shimadzu model TG-50 (Shimadzu Corp., Kyoto, Japan) equipment operating with nitrogen atmosphere with a heating rate of 10 ◦C/min in a temperature range of 25 to 600 ◦C. The TG/DTG analysis followed the ASTM E1131 standard [35].

2.8. Differential Scanning Calorimetry (DSC) Polymers 2020, 12, x 6 of 20 For differential scanning calorimetry (DSC) analysis the composites and fibers were crushed and placedequipment in an was aluminum used. The crucible. DSC analys A Shimadzuis was carried model out DSC-60A under nitrogen Plus (Shimadzu atmosphere Corp., with Kyoto, a flow Japan) rate equipmentof 50 mL/min, was heating used. The rate DSC of 10 analysis °C/min, was in the carried temperature out under range nitrogen of 25 atmosphere to 600 °C. with a flow rate of 50 mL/min, heating rate of 10 ◦C/min, in the temperature range of 25 to 600 ◦C. 2.9. Dynamic Mechanical Analysis (DMA) 2.9. Dynamic Mechanical Analysis (DMA) Dynamic mechanical analysis was carried out in order to identify important parameters such as glassDynamic transition mechanical temperature, analysis as well was as carried to inve outstigate in order the to viscoelastic identify important behavior parameters of composites. such asSpecimens glass transition with 0, temperature,10, 20, 30, and as 40 well vol% as of to carn investigateauba fibers the viscoelasticwere fabricated behavior according of composites. to ASTM SpecimensD4065 standard with [36] 0, 10, and 20, the 30, test and mode 40 vol% was three of carnauba points bending fibers were for specimens fabricated fixed according by one to end. ASTM The D4065equipment standard used [was36] anda model the test DMA mode Q800, was TA three Instru pointsments bending (New forCastle, specimens DE, USA), fixed operated by one end.at a Theheating equipment rate of used5 °C/min was and a model specimens DMA Q800,dimensions TA Instruments of 65 × 12 (New × 3 mm, Castle, from DE, which USA), the operated curves atof astorage heating modulus, rate of 5 lossC/ minmodulus, and specimens and tangent dimensions delta were of recorded. 65 12 3 mm, from which the curves of ◦ × × storage modulus, loss modulus, and tangent delta were recorded. 3. Results and Discussion 3. Results and Discussion 3.1. Pullout Test 3.1. Pullout Test Figure 5 presents the results obtained in pullout tests for the different depths of carnauba fiber Figure5 presents the results obtained in pullout tests for the di fferent depths of carnauba fiber embedded in the epoxy resin. The graph represents the pullout stress as a function of the fiber embedded in the epoxy resin. The graph represents the pullout stress as a function of the fiber embedded length. embedded length.

Figure 5. Pullout test of the different depths of carnauba fiber embedding in the epoxy resin. Figure 5. Pullout test of the different depths of carnauba fiber embedding in the epoxy resin.

The results observed in Figure 5 follow the model proposed by Kelly and Tysson [28], being associated with two straight lines that intersect at the critical length (Lc) of the carnauba fiber in relation to the epoxy matrix. The upper straight line with the lowest slope represents the linear adjustment between the maximum tensile stress values in pullout observed in the fiber embedded length range of 7.5 to 30 mm. A stress value of approximately 65 to 110 MPa, was found, which is relatively lower as compared to the range of tensile strength, 205–264 MPa, shown for the carnauba fiber in Table 1 [26]. This might be a consequence of the variability of carnauba fibers, as any NLF [4]. The linear adjustments applied to the pullout stresses of the carnauba fiber/epoxy are represented by Equation (2) with higher slope curve and Equation (3) with lower slope curve. 𝜎 = 8.51 𝐿 22.08 (2)

𝜎 = 0.106 𝐿 79.17 (3)

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The results observed in Figure5 follow the model proposed by Kelly and Tysson [ 28], being associated with two straight lines that intersect at the critical length (Lc) of the carnauba fiber in relation to the epoxy matrix. The upper straight line with the lowest slope represents the linear adjustment between the maximum tensile stress values in pullout observed in the fiber embedded length range of 7.5 to 30 mm. A stress value of approximately 65 to 110 MPa, was found, which is relatively lower as compared to the range of tensile strength, 205–264 MPa, shown for the carnauba fiber in Table1[ 26]. This might be a consequence of the variability of carnauba fibers, as any NLF [4]. The linear adjustments applied to the pullout stresses of the carnauba fiber/epoxy are represented by Equation (2) with higher slope curve and Equation (3) with lower slope curve.

σ = 8.51 L + 22.08 (2)

σ = 0.106 L + 79.17 (3)

It is important to note that, for embedding values lower than Lc = 6.79 mm, the fibers were removed (pulled out) from the matrix, while for a higher value, an almost constant linear relation of the tensile strength with the embedded length, occurs around 65–110 MPa. In this case, when the maximum pullout stress is reached, the fiber breaks without being removed from the epoxy matrix. The interfacial resistance of the carnauba fiber in relation to the epoxy matrix was evaluated by the equation of Kelly and Tyson [28]. dσ f τi = (4) 2Lc where “d” is the equivalent diameter of the fiber and σf is the tensile strength of the fiber. With the value of Lc = 6.79 mm it was possible to calculate the value of τi through Equation (4), considered as the interfacial shear resistance between the fibers and the matrix [29]. The average diameter found for the carnauba fibers of 0.769 mm was used together with the average stress value of 65.17 MPa, to obtain 3.69 MPa for the carnauba fiber/epoxy matrix interfacial shear strength. The obtained value is considered relatively low. This fact may be a consequence of the hydrophilic nature of the fibers, which contributes to the reduction of the interfacial interaction between the fiber/matrix. However, the value (τi = 3.69 MPa) herein obtained is relatively higher than the epoxy-coconut fiber interaction (τi = 1.42 MPa) and very close to that for the epoxy-PALF fiber interaction (τi = 4.93 MPa) reported by Da Luz et al. [37]. The authors attributed these low values to the chemical nature of the NLFs together with the low surface roughness presented by their fibers. After the pullout test, the specimens were analyzed by SEM to obtain relevant information on the interaction between the fiber/matrix. As an example, the specimen with the embedded length of 1 mm, in which the fiber was removed by pullout from the matrix, is illustrated in Figure6. In Figure6a, one clearly sees the hole left in the epoxy after the pulling out of the carnauba fiber. With the increasing magnification in Figure6b, it is possible to note that remnants of the carnauba fiber still adhered to the epoxy matrix. These remnants can be associated with the microfibrils that were still better adhered to the matrix. In fact, after the fiber was removed, they remained attached to the epoxy resin. Polymers 2020, 12, x 7 of 20

It is important to note that, for embedding values lower than Lc = 6.79 mm, the fibers were removed (pulled out) from the matrix, while for a higher value, an almost constant linear relation of the tensile strength with the embedded length, occurs around 65–110 MPa. In this case, when the maximum pullout stress is reached, the fiber breaks without being removed from the epoxy matrix. The interfacial resistance of the carnauba fiber in relation to the epoxy matrix was evaluated by the equation of Kelly and Tyson [28].

𝑑𝜎 𝜏 = (4) 2𝐿

where “d” is the equivalent diameter of the fiber and σf is the tensile strength of the fiber. With the value of Lc = 6.79 mm it was possible to calculate the value of τi through Equation (4), considered as the interfacial shear resistance between the fibers and the matrix [29]. The average diameter found for the carnauba fibers of 0.769 mm was used together with the average stress value of 65.17 MPa, to obtain 3.69 MPa for the carnauba fiber/epoxy matrix interfacial shear strength. The obtained value is considered relatively low. This fact may be a consequence of the hydrophilic nature of the fibers, which contributes to the reduction of the interfacial interaction between the fiber/matrix. However, the value (τi = 3.69 MPa) herein obtained is relatively higher than the epoxy-coconut fiber interaction (τi = 1.42 MPa) and very close to that for the epoxy-PALF fiber interaction (τi = 4.93 MPa) reported by Da Luz et al. [37]. The authors attributed these low values to the chemical nature of the NLFs together with the low surface roughness presented by their fibers. After the pullout test, the specimens were analyzed by SEM to obtain relevant information on Polymers 2020, 12, 2090 8 of 20 the interaction between the fiber/matrix. As an example, the specimen with the embedded length of 1 mm, in which the fiber was removed by pullout from the matrix, is illustrated in Figure 6.

Figure 6. Specimens micrographs micrographs of of pullout pullout tests. tests. (a ()a 200×,) 200 (b,() b800×.) 800 . × × 3.2. TensileIn Figure Test 6a, one clearly sees the hole left in the epoxy after the pulling out of the carnauba fiber. WithFigure the increasing7 shows magnification stress-strain in curves Figure obtained 6b, it is possible in the tensileto note that tests remnants for the neatof the epoxy carnauba and thefiberPolymers composites still 2020 adhered, 12, withx to 10,the 20, epoxy 30, andmatrix. 40 vol% These of remnan carnaubats can fibers. be associated with the microfibrils8 of 20that were still better adhered to the matrix. In fact, after the fiber was removed, they remained attached to the epoxy resin.

3.2. Tensile Test Figure 7 shows stress-strain curves obtained in the tensile tests for the neat epoxy and the composites with 10, 20, 30, and 40 vol% of carnauba fibers.

Figure 7. Stress-strain curves for the neat epoxy resin and composites reinforced with carnauba fibers. Figure 7. Stress-strain curves for the neat epoxy resin and composites reinforced with carnauba fibers. (a) Neat epoxy, (b) 10, (c) 20, (d) 30, and (e) 40 vol%. (a) Neat epoxy, (b) 10, (c) 20, (d) 30, and (e) 40 vol%. Table2 presents the average results for the mechanical properties for the neat epoxy and composites Table 2 presents the average results for the mechanical properties for the neat epoxy and withcomposites different with volume different fractions volume ofcarnauba fractions fibers.of carnauba Figure fibers.8 shows Figure the graphs8 shows corresponding the graphs to thecorresponding results of tensile to the strength, results elasticof tensile modulus, strength, and elastic elongation modulus, presented and elongation in Table presented2. The tensile in Table strength values2. The obtained tensile strength for the values DGEBA-TETA obtained for epoxy the DG resinEBA-TETA are comparable epoxy resin with are values comparable already with consolidated values inalready the literature consolidated [2,23 ,in27 the]. literature The results [2,23,27]. show The relatively results show superior relatively tensile superior properties tensile for properties composites reinforcedfor composites with di reinforcedfferent volume with different fractions volume of carnauba fractions fibers. of carnauba In fact,the fibers. values In fact, of tensile the values strength of for alltensile composites strength are for higher all composites than that forare thehigher neat than epoxy that resin. for the Therefore, neat epoxy the carnaubaresin. Therefore, fiber causes the an effcarnaubaective reinforcement fiber causes toan the effective epoxy reinforcement matrix. For instance, to the epoxy the 40 matrix. vol% carnauba For instance, fiber compositethe 40 vol% is 40% strongercarnauba than fiber the composite neat epoxy. is 40% stronger than the neat epoxy.

Table 2. Mechanical properties for the epoxy resin and carnauba fibers reinforced composites.

Total Strain Volume Fraction (%) Tensile Strength (MPa) Elastic Modulus (GPa) (% of elongation) 0 29.3 ± 5.7 1.66 ± 0.48 1.1 ± 0.2 10 37.8 ± 3.2 2.29 ± 0.07 1.5 ± 0.1 20 37.7 ± 2.2 2.57 ± 0.10 1.4 ± 0.1 30 39.3 ± 5.1 2.48 ± 0.29 1.5 ± 0.2 40 40.9 ± 5.8 2.80 ± 0.37 1.5 ± 0.3

According to Table 2, a tendency of increasing the composites elastic modulus with the carnauba fiber volumetric fraction is also observed. Indeed, the 40 vol% carnauba fiber composite is 69% stiffer than the neat epoxy. The composites reinforced with carnauba fibers present total strain superior (~36%) to neat epoxy resin. On the other hand, the values observed between the composites did not show any significant variation within the standard deviation. The data presented in Table 2 highlight for the first time the reinforcement in a polymer matrix by carnauba fibers. A practical point regarding this reinforcement effect caused by carnauba fiber is the possibility of replacing a commonly used glass fiber reinforced composite in engineering applications. In fact, a comparison with DGEBA/TETA epoxy composite reinforced with 30 vol% of glass fiber [38] reveals that its specific (density-rationalized) tensile strength of 64.5 MPa.cm3/g is only 40% higher than that of 35.4 MPa.cm3/g for the 30 vol% carnauba fiber epoxy composite. By

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Table 2. Mechanical properties for the epoxy resin and carnauba fibers reinforced composites.

Total Strain Volume Fraction (%) Tensile Strength (MPa) Elastic Modulus (GPa) (% of elongation) 0 29.3 5.7 1.66 0.48 1.1 0.2 ± ± ± 10 37.8 3.2 2.29 0.07 1.5 0.1 ± ± ± Polymers 2020, 2012, x 37.7 2.2 2.57 0.10 1.4 0.1 9 of 20 ± ± ± 30 39.3 5.1 2.48 0.29 1.5 0.2 ± ± ± considering40 the aforementioned 40.9environmental5.8 and cost effectiveness 2.80 0.37 advantages, 1.5the carnauba0.3 ± ± ± composites might be a viable substitute for fiberglass.

Figure 8. Mechanical properties as a function of the volume fraction for the neat epoxy resin and Figure 8. Mechanical properties as a function of the volume fraction for the neat epoxy resin and a b c carnaubacarnauba fibers fibers reinforced reinforced epoxy epoxy composites. composites. ( ()a Tensile) Tensile strength, strength, ( )(b elastic) elastic modulus, modulus, and and ( ) ( totalc) total strain. strain. According to Table2, a tendency of increasing the composites elastic modulus with the carnauba fiber3.3. volumetric Izod Impact fractionTest is also observed. Indeed, the 40 vol% carnauba fiber composite is 69% stiffer than the neat epoxy. The composites reinforced with carnauba fibers present total strain superior The results of the average Izod impact energy for the neat DGEBA-TETA epoxy resin and (~36%) to neat epoxy resin. On the other hand, the values observed between the composites did not composites with different volume fractions of carnauba fibers are presented in Table 3, and show any significant variation within the standard deviation. graphically shown in Figure 9. The standard deviation value listed for the epoxy resin is relatively The data presented in Table2 highlight for the first time the reinforcement in a polymer matrix low when compared to the values associated with the composites. This fact is expected, since the by carnauba fibers. A practical point regarding this reinforcement effect caused by carnauba fiber NLFs present a high variability of their properties [4] and the increase of concentration in the iscomposites, the possibility causes of oscillation replacing in a th commonlyeir mechanical used properties glass fiber [38–41]. reinforced composite in engineering applications. In fact, a comparison with DGEBA/TETA epoxy composite reinforced with 30 vol% of 3 glass fiberTable [38 3.] Results reveals of thatthe Izod its specific impact test (density-rationalized) for epoxy matrix composites tensile reinforced strength with of 64.5 continuous MPa.cm and/ g is only 40% higheraligned than carnauba that of fibers. 35.4 MPa.cm3/g for the 30 vol% carnauba fiber epoxy composite. By considering the aforementioned environmental and cost effectiveness advantages, the carnauba composites might Fiber Fraction (vol%) Average Izod Absorbed Energy (J/m) be a viable substitute for fiberglass.0% 21.5 ± 4.9 10% 65.0 ± 13.0 20% 80.4 ± 21.0 30% 137.4 ± 37.7 40% 201.9 ± 53.8

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3.3. Izod Impact Test The results of the average Izod impact energy for the neat DGEBA-TETA epoxy resin and composites with different volume fractions of carnauba fibers are presented in Table3, and graphically shown in Figure9. The standard deviation value listed for the epoxy resin is relatively low when compared to the values associated with the composites. This fact is expected, since the NLFs present a high variability of their properties [4] and the increase of concentration in the composites, causes oscillation in their mechanical properties [38–41].

Table 3. Results of the Izod impact test for epoxy matrix composites reinforced with continuous and aligned carnauba fibers.

Fiber Fraction (vol%) Average Izod Absorbed Energy (J/m) 0% 21.5 4.9 ± 10% 65.0 13.0 ± 20% 80.4 21.0 ± 30% 137.4 37.7 Polymers 2020, 12, x ± 10 of 20 40% 201.9 53.8 ±

Figure 9. Izod impact energy as a function of the fiber fraction for the neat epoxy resin and carnauba fibersFigure reinforced 9. Izod impact composites. energy as a function of the fiber fraction for the neat epoxy resin and carnauba fibers reinforced composites. All specimens subjected to the impact test suffered a complete rupture, validating the results obtainedAll specimens [41]. Figure subjected 10 shows to the the aspect impact of test the suffer brokened specimens a complete after rupture, the impact validating test. Thethe results visual analysisobtained of [41]. the Figure specimens 10 shows after rupturethe aspect confirms of the thebroken occurrence specimens of a fragileafter the fracture impact in test. the specimensThe visual ofanalysis pure epoxyof the resin,specimens presenting after rupture a smooth confirms and mirror the occurrence surface [42 of]. a Asfragile the volumefracture fractionin the specimens of fibers increases,of pure epoxy it is possibleresin, presenting to observe a smooth a greater and irregularity mirror surface in the [42]. fracture As the surface, volume giving fraction evidence of fibers to theincreases, transition it is frompossible fragile to observe to ductile-fragile a greater irregularity behavior. in the fracture surface, giving evidence to the transitionWith thefrom data fragile obtained to ductile-fragile from the ANOVA behavior. presented in Table4, the hypothesis of equality between the averages at the 5% significance level is rejected, because Fc = 74.06 is higher than the F critical (tabulated) = 2.496. Therefore, the volume fraction of carnauba fibers in the composites has a direct effect on the Izod impact energy.

Figure 10. Fractured specimens after the Izod test: (a) 0, (b) 10, (c) 20, (d) 30, and (e) 40 vol% of carnauba fibers.

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Figure 9. Izod impact energy as a function of the fiber fraction for the neat epoxy resin and carnauba fibers reinforced composites.

All specimens subjected to the impact test suffered a complete rupture, validating the results obtained [41]. Figure 10 shows the aspect of the broken specimens after the impact test. The visual analysis of the specimens after rupture confirms the occurrence of a fragile fracture in the specimens of pure epoxy resin, presenting a smooth and mirror surface [42]. As the volume fraction of fibers increases,Polymers 2020 it, 12is, possible 2090 to observe a greater irregularity in the fracture surface, giving evidence 11to ofthe 20 transition from fragile to ductile-fragile behavior.

Figure 10. FracturedFractured specimens specimens after after the the Izod Izod test: test: (a (a) )0, 0, ( (bb) )10, 10, ( (cc)) 20, 20, ( (dd)) 30, 30, and and ( (ee)) 40 40 vol% vol% of of carnaubacarnauba fibers. fibers.

Table 4. Variance analysis of average impact energies obtained for reinforced epoxy matrix composites with percentages of carnauba fibers from 0 to 40 vol%.

Sum of Variation Causes DF Mean Square F(calc.) F Critical (tab.) Squares Treatments 4 312,767.02 78,191.75 74.06 2.496 Residue 75 79,185.15 1,055.80 Total 79 391,952.17

In order to verify which fraction of fibers presented the best results of Izod impact energy, the Tukey test was applied to compare performances with a 95% confidence level. The lower significant difference (LSD) was found as 34.04. The comparison data between the averages of Izod impact energy between the volume fractions of carnauba fibers are shown in Table5.

Table 5. Results obtained for the differences (LSD) between the average values of the Izod impact energy, in the volume fractions of carnauba fibers from 0 to 30 vol%, after application of the Tukey test.

Vol.% 0 10 20 30 40 0 0 43.45 58.85 115.93 180.37 10 43.45 0 15.40 72.48 136.92 20 58.85 15.40 0 57.08 121.52 30 115.93 72.48 57.08 0 64.44 40 180.37 136.92 121.52 64.44 0

Based on the results in Table5, it was found that, with a 95% confidence level, the composite with 40 vol% of carnauba fibers, presented the best performance, exhibiting a higher average energy Polymers 2020, 12, 2090 12 of 20 value of Izod impact (201.9 J/m). This proves to be a superior value in comparison with the other volume fractions of fibers, since the differences obtained are higher than the calculated LSD (34.04). It is important to note that there is no significant difference between the average Izod impact energy values of the carnauba fiber reinforcement with 10 and 20 vol%. Indeed, the values in Table5 for the averages is not greater than the calculated LSD. This suggests that a least 30 vol% of carnauba fiber must be incorporated into the epoxy matrix for an effective reinforcement. The increase in impact energy with the increase amount of carnauba fibers in the composite may be related to the fracture mechanisms acting for composites of 20, 30, and 40 vol% of fibers. In order to confirm and better understand the evolution of the fracture mechanisms acting on the tested materials, PolymersSEM images 2020, 12 of, x the fracture surfaces of the specimens are shown in Figure 11. 12 of 20

Figure 11. Scanning electron microscopy of fracture surfaces of composites reinforced with carnauba Figure 11. Scanning electron microscopy of fracture surfaces of composites reinforced with carnauba fibers after Izod impact tests. (a) 0 vol% 400 ;(b) 10 vol% 400 ;(c) 20 vol% 600 ;(d) 30 vol% 400 ; fibers after Izod impact tests. (a) 0 vol% 400×; ×(b) 10 vol% 400×; (×c) 20 vol% 600×; (d×) 30 vol% 400×; and× and (e) 40 vol% 300 of fibers. (e) 40 vol% 300× of fibers.×

In this figure, it is possible to identify several fracture mechanisms acting on the composites. In Figure 11a, the mechanism of fragile fracture is clearly identified, due to the “river marks” present on the impact surface of the neat epoxy specimen. The same phenomenon can be observed for the composites with 10 vol% of carnauba fibers shown in Figure 11b, presenting a low effective reinforcement by the fibers for the composites with this volumetric fraction [43]. For the composites with 20 vol% of fibers, Figure 11c, the pullout of the fibers in the matrix is observed, evidenced by the circular holes shown in the fractography. For the 30 and 40 vol% composites, Figure 11d and Figure 11e respectively, a more effective performance of the fibers is verified, in which the rupture and detachment of the fibers in the matrix occurred. This mechanism can be related to the absorption of high impact energies, Table 3 and Figure 9, associated with these higher volume fractions of carnauba fibers.

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In this figure, it is possible to identify several fracture mechanisms acting on the composites. In Figure 11a, the mechanism of fragile fracture is clearly identified, due to the “river marks” present on the impact surface of the neat epoxy specimen. The same phenomenon can be observed for the composites with 10 vol% of carnauba fibers shown in Figure 11b, presenting a low effective reinforcement by the fibers for the composites with this volumetric fraction [43]. For the composites with 20 vol% of fibers, Figure 11c, the pullout of the fibers in the matrix is observed, evidenced by the circular holes shown in the fractography. For the 30 and 40 vol% composites, Figure 11d,e respectively, a more effective performance of the fibers is verified, in which the rupture and detachment of the fibers in the matrix occurred. This mechanism can be related to thePolymers absorption 2020, 12, x of high impact energies, Table3 and Figure9, associated with these higher volume13 of 20 fractions of carnauba fibers. TheThe incorporationincorporation of carnauba carnauba fibers fibers in in composites composites can can be bedirectly directly linked linked to the to increase the increase in the in theimpact impact energy energy presented presented by these by these materials. materials. This fact This isfact associated is associated with the with effect the of e ffcarnaubaect of carnauba fibers fibersas a means as a means of interruption of interruption or deviation or deviation of the of crack’s the crack’s propagation propagation direction, direction, which which contribute contribute to a to agreater greater rupture rupture surface surface area. area. Therefore, Therefore, this this causes causes an an increase increase in in the the absorbed absorbed impact impact energy energy [27]. [27 ]. TheThe important important pointpoint regardingregarding the impact impact results results is is that that the the carnauba carnauba fiber fiber might might indeed indeed promote promote a areinforcement reinforcement effect effect in in polymer polymer matrix matrix composites. composites.

3.4.3.4. Fourier Fourier TransformTransform InfraredInfrared SpectroscopySpectroscopy (FTIR)(FTIR) TheThe FTIR FTIR spectra spectra are are presented presented in Figure in Figure 12, where 12, where it is possible it is possible to retrieve to important retrieve informationimportant regardinginformation the regarding chemical structure the chemical presented structure by thepresented carnauba by fiberthe carnauba and its interaction fiber and its with interaction the epoxy with resin. the epoxy resin.

FigureFigure 12. 12.General General FourierFourier transformtransform infrared spectroscopy spectroscopy (FTIR) (FTIR) spectra spectra for for carnauba carnauba fiber, fiber, epoxy epoxy resin,resin, and and composites composites ( (aa).). DetailsDetails for epoxy resin resin and and carnauba carnauba fibers fibers (b (b) )and and for for the the epoxy epoxy composites composites (c().c).

TheThe FTIR FTIR spectrumspectrum forfor thethe carnaubacarnauba fiber fiber shown in in Figure Figure 12a12a reveals reveals an an absorption absorption band band at at 1 1 33903390 cm cm−−1, which is is attributed attributed to to the the extension extension of ofthe the OH OH bond bond [44]. [44 Bands]. Bands at 2890 at 2890 and and28502850 cm−1 cmare− areattributed attributed to CH to CH2 vibrations,2 vibrations, related related to molecules to molecules present present in cellulose in cellulose and hemicellulose and hemicellulose [45]. The [45 ]. 1 Thebands bands observed observed in 1653 in 1653 and and 1737 1737 cm cm−1, −Figure, Figure 12b, 12 b,correspond correspond to toC=O C= Obonds, bonds, probably probably due due to to functional groups (carboxylic acids, aliphatic, and ketones) present in lignin and hemicellulose [46,47]. The bands located between 1605 to 1505 cm−1 can be attributed to vibrations in aromatic rings [46], and in 1250 cm−1 refers to the stretching of the CO bonds of phenolic groups present in the fibers constituents. The alcohols present in the constitution of cellulose show vibrations of deformation associated with the OH bond and generally appear around 1360 cm−1 [45]. The increase in the volume fraction of fibers in the composite causes the variation in the relative absorption of the 1737 cm−1 band shown in Figure 12c. However, there is no significant absorption in this wavelength in the spectrum of the DGEBA-TETA resin in Figure 12b. This vibration is characteristic of C=O of functional groups present in the constituents of the fibers. As such, the proportional increase in the volume fraction of carnauba fiber in the composites is associated with an

Polymers 2020, 12, 2090 14 of 20 functional groups (carboxylic acids, aliphatic, and ketones) present in lignin and hemicellulose [46,47]. 1 The bands located between 1605 to 1505 cm− can be attributed to vibrations in aromatic rings [46], 1 and in 1250 cm− refers to the stretching of the CO bonds of phenolic groups present in the fibers constituents. The alcohols present in the constitution of cellulose show vibrations of deformation 1 associated with the OH bond and generally appear around 1360 cm− [45]. The increase in the volume fraction of fibers in the composite causes the variation in the relative 1 absorption of the 1737 cm− band shown in Figure 12c. However, there is no significant absorption in this wavelength in the spectrum of the DGEBA-TETA resin in Figure 12b. This vibration is characteristic of C=O of functional groups present in the constituents of the fibers. As such, the proportional increase inPolymers the volume 2020, 12 fraction, x of carnauba fiber in the composites is associated with an increase in the absorption14 of 20 of this band. One might speculate that this increase interferes with the surface adhesion between theincrease hydrophilic in the absorption carnauba fiber of this and band. the hydrophobic One might speculate epoxy resin. that this increase interferes with the surface adhesion between the hydrophilic carnauba fiber and the hydrophobic epoxy resin. 3.5. ThermoGravimetric Analysis (TG/DTG) 3.5. ThermoGravimetric Analysis (TG/DTG) As shown in Figure 13, the epoxy resin is thermally stable up to approximately 280 ◦C, with As shown in Figure 13, the epoxy resin is thermally stable up to approximately 280 °C, with a a negligible mass loss of 1.05%, Figure 13b. At approximately 300 ◦C, the greatest mass loss of the epoxy negligible mass loss of 1.05%, Figure 13b. At approximately 300 °C, the greatest mass loss of the epoxy begins, at a maximum rate of 380 ◦C and extending up to 490 ◦C. This mass loss represents about 80.1%begins, of at the a maximum mass of the rate sample, of 380 which °C and can extending be associated up to with 490 °C. the This degradation mass loss of represents the polymer about chains of80.1% the epoxyof the mass resin. of The the sample, TG curves which obtained can be forasso theciated epoxy-fiber with the degradation composites of with the 10,polymer 20, 30, chains 40 vol% of the epoxy resin. The TG curves obtained for the epoxy-fiber composites with 10, 20, 30, 40 vol% fractions in comparison with the carnauba fiber and the DGEBA-TETA revealed an intermediate fractions in comparison with the carnauba fiber and the DGEBA-TETA revealed an intermediate thermal stability in Figure 13a. thermal stability in Figure 13a.

Figure 13. Thermogravimetric curves for carnauba fibers, resin, epoxy, and composites (a); carnauba Figure 13. Thermogravimetric curves for carnauba fibers, resin, epoxy, and composites (a); carnauba fibers and epoxy resin (b). fibers and epoxy resin (b). Table6 presents the main thermogravimetric parameters, temperatures, and mass loss for the plain Table 6 presents the main thermogravimetric parameters, temperatures, and mass loss for the carnauba fiber, neat epoxy (0% fiber), and carnauba fiber composites with 10, 20, 30, 40 vol% fractions. plain carnauba fiber, neat epoxy (0% fiber), and carnauba fiber composites with 10, 20, 30, 40 vol% The results in Table6 indicate that the composites present better thermal stability in relation to fractions. the plain carnauba fibers. Indeed, both the initial degradation (267 ◦C) and maximum degradation rate (353Table◦C) 6. ofThermogravimetric the plain carnauba parameters fiber are for lower the thanneat theepoxy, corresponding plain fiber, temperaturesand carnauba (326fiber ◦C) and (360 ◦composites.C), respectively, for the 10 vol% carnauba fiber composites. However, these temperatures for all composites are slightly lower than those corresponding to the neat epoxy. Similar results were found Mass Temperature of Mass Loss at Mass forCarnauba other less known NFLs polymerInitial composites [48]. The composites present an average mass loss of Loss up Maximum the End of Loss at 67%Fiber at the end of the second stageDegradation of degradation, thus showing better thermal stability than epoxy to 200 °C Degradation Rate Second Stage 600 °C composites(vol %) reinforced withTemperature a glass fiber treated(°C) with graphene oxide (GO), reduced graphene oxide (%) (°C) (%) (%) (rGO), and graphene nanoplatelets (GNPs) [49]. Based on these results, a thermal stability limit of 300 Plain fiber 9.63 267.3 353.1 63.1 74.7 C could be considered for carnauba fiber composites. ◦ Neat 1.05 341.6 380.7 81.9 86.1 epoxy 10 1.49 326.4 360.1 66.5 73.4 20 2.50 320.5 355.6 63.3 72.8 30 3.75 325.9 368.5 75.6 81.9 40 1.75 305.3 350.7 63.2 71.9

The results in Table 6 indicate that the composites present better thermal stability in relation to the plain carnauba fibers. Indeed, both the initial degradation (267 °C) and maximum degradation rate (353 °C) of the plain carnauba fiber are lower than the corresponding temperatures (326 °C) and (360 °C), respectively, for the 10 vol% carnauba fiber composites. However, these temperatures for all composites are slightly lower than those corresponding to the neat epoxy. Similar results were

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Table 6. Thermogravimetric parameters for the neat epoxy, plain fiber, and carnauba fiber composites.

Initial Temperature Mass Loss at Carnauba Mass Loss up Degradation of Maximum the End of Mass Loss at Fiber to 200 C (%) Temperature Degradation Second Stage 600 C (%) (vol %) ◦ ◦ (◦C) Rate (◦C) (%) PolymersPlain 2020 fiber, 12, x 9.63 267.3 353.1 63.1 74.715 of 20 Neat epoxy 1.05 341.6 380.7 81.9 86.1 found for10 other less known 1.49 NFLs polymer 326.4 composites [48]. 360.1 The composites pr 66.5esent an average 73.4 mass loss of 67%20 at the end of 2.50 the second stage 320.5 of degradation, thus 355.6 showing better 63.3 thermal stability 72.8 than epoxy composites30 reinforced 3.75 with a glass 325.9 fiber treated with 368.5 graphene oxide (GO), 75.6 reduced graphene 81.9 oxide (rGO),40 and graphene 1.75 nanoplatelets 305.3(GNPs) [49]. Based 350.7 on these results, 63.2a thermal stability 71.9 limit of 300 °C could be considered for carnauba fiber composites. 3.6. Differential Scanning Calorimetry (DSC) 3.6. Differential Scanning Calorimetry (DSC) Figure 14 shows the DSC curves obtained for the epoxy resin, the carnauba fiber, and the studied Figure 14 shows the DSC curves obtained for the epoxy resin, the carnauba fiber, and the studied composites. It is possible to observe an endothermic peak at 63 ◦C, associated with the glass transition composites. It is possible to observe an endothermic peak at 63 °C, associated with the glass transition temperature (Tg) of the epoxy resin, as highlighted in Figure 14a [50]. As for the endothermic temperature (Tg) of the epoxy resin, as highlighted in Figure 14a [50]. As for the endothermic peak of peak of the plain carnauba fiber at 107 ◦C, it is attributed to the release of moisture adhered to the plain carnauba fiber at 107 °C, it is attributed to the release of moisture adhered to the hydrophilic the hydrophilic surface. surface.

Figure 14. Differential scanning calorimetry (DSC) curves for epoxy resin and carnauba fibers (a) and (Figureb) composites. 14. Differential scanning calorimetry (DSC) curves for epoxy resin and carnauba fibers (a) and (b) composites. The increase of the volume fraction of carnauba in the epoxy matrix causes the displacement The increase of the volume fraction of carnauba in the epoxy matrix causes the displacement of of the endothermic peak (63 ◦C) observed in the DSC curve for the neat epoxy. The composites the endothermic peak (63 °C) observed in the DSC curve for the neat epoxy. The composites endothermic peaks in Figure 14b are probably related to both the release of moisture present in endothermic peaks in Figure 14b are probably related to both the release of moisture present in the the carnauba fiber and the Tg of the epoxy matrix. This may explain the increase in the endothermic carnauba fiber and the Tg of the epoxy matrix. This may explain the increase in the endothermic peaks peaks temperature (75–103 C) with the increase in the amount of fibers in the composite, in spite of temperature (75–103 °C) with◦ the increase in the amount of fibers in the composite, in spite of the the well-known interference of NLFs with the polymer matrix crystalline arrangement [50,51]. well-known interference of NLFs with the polymer matrix crystalline arrangement [50,51]. 3.7. Dynamic Mechanical Analysis (DMA) 3.7. Dynamic Mechanical Analysis (DMA) Table7 and Figure 15 show the curves of storage modulus (E’), loss modulus (E”), and tangent Table 7 and Figure 15 show the curves of storage modulus (E’), loss modulus (E”), and tangent δ deltadelta (tan(tan δ)) of the neat neat epoxy epoxy [52] [52 ]and and different different carnau carnaubaba fiber fiber composites, composites, from from 25 up 25 upto 200 to 200°C. ◦C.

Table 7. Dynamic mechanical analysis (DMA) parameters.

Material E’ (35°C) E” Tg (°C) Tan δ Tg (°C) Reference 0 1352 64 72 [52–54] 10 2963 99 82 20 2848 74 83 PW 1 30 3044 69 86

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Table 7. Dynamic mechanical analysis (DMA) parameters.

Material E’ (35◦C) E” Tg (◦C) Tan δ Tg (◦C) Reference 0 1352 64 72 [52–54] 10 2963 99 82 20 2848 74 83 PW 1 Polymers 2020, 12, x30 3044 69 86 16 of 20 40 2674 81 84 40 2674 1 PW: 81 Present Work. 84 1 PW: Present Work.

Figure 15. Dynamic mechanical analysis. (a) Storage modulus, (b) loss modulus, and (c) tangent delta Figure 15. Dynamic mechanical analysis. (a) Storage modulus, (b) loss modulus, and (c) tangent delta for the carnauba fiber composites. for the carnauba fiber composites. The E’ results in Figure 15a reveal an improvement of the storage modulus with the incorporation The E’ results in Figure 15a reveal an improvement of the storage modulus with the of carnauba fiber, which confirms the reinforcement effect owing to its better interaction with the epoxy incorporation of carnauba fiber, which confirms the reinforcement effect owing to its better matrix [53]. As for the E” results in Figure 15b, the incorporation of carnauba fiber displaces the loss interaction with the epoxy matrix [53]. As for the E” results in Figure 15b, the incorporation of modulus peaks to higher temperatures, as compared to that of the neat epoxy reported elsewhere [52,54]. carnauba fiber displaces the loss modulus peaks to higher temperatures, as compared to that of the The E” peak is related to the structural relaxation and might be assigned to T [41]. A similar neat epoxy reported elsewhere [52,54]. g situation occurs for the tan δ peaks in Figure 15c. A comparison between the value of Tg for the neat The E” peak is related to the structural relaxation and might be assigned to Tg [41]. A similar epoxy obtained from DSC (63 ◦C) is close to that from DMA tan δ (74 ◦C). As for the Tg of the composites situation occurs for the tan δ peaks in Figure 15c. A comparison between the value of Tg for the neat (82–84 ◦C), they are slightly higher due to the carnauba fiber interference with the mobility of epoxy epoxy obtained from DSC (63 °C) is close to that from DMA tan δ (74 °C). As for the Tg of the macromolecular chains [52]. composites (82–84 °C), they are slightly higher due to the carnauba fiber interference with the 4.mobility Summary of epoxy and Conclusions macromolecular chains [52].

4. SummaryPullout testsand Conclusions provided a critical embedded length of 6.79 mm for the carnauba fiber in the DGEBA/TETA epoxy resin with an interface shear strength value of 3.69 MPa, which is comparable Pullout tests provided a critical embedded length of 6.79 mm for the carnauba fiber in the to those presented by other NLFs. DGEBA/TETA epoxy resin with an interface shear strength value of 3.69 MPa, which is comparable Epoxy composites reinforced with carnauba fibers showed higher tensile strength values to those presented by other NLFs. (37.8–40.9 MPa) than the neat epoxy (29.3 MPa), characterizing a reinforcement effect. There was also Epoxy composites reinforced with carnauba fibers showed higher tensile strength values (37.8– 40.9 MPa) than the neat epoxy (29.3 MPa), characterizing a reinforcement effect. There was also a tendency of increasing the composites elastic modulus (2.29 to 2.80 GPa) as the volume fraction increases, which was attributed to the higher stiffness of the carnauba fiber. The total strain (1.41.5%) did not show any significant variation between the different carnauba fiber composites, but is superior to the neat epoxy resin (1.1%). Izod impact tests revealed an increase in impact energy with the volume fraction of carnauba fibers incorporated in the epoxy resin. The maximum value obtained for the Izod impact energy of

Polymers 2020, 12, 2090 17 of 20 a tendency of increasing the composites elastic modulus (2.29 to 2.80 GPa) as the volume fraction increases, which was attributed to the higher stiffness of the carnauba fiber. The total strain (1.41.5%) did not show any significant variation between the different carnauba fiber composites, but is superior to the neat epoxy resin (1.1%). Izod impact tests revealed an increase in impact energy with the volume fraction of carnauba fibers incorporated in the epoxy resin. The maximum value obtained for the Izod impact energy of 201.9 J/m, for the percentage of 40 vol% carnauba fibers is more than nine times that of the neat epoxy of 21.5 J/m. The ANOVA confirmed the highest Izod impact energy results for the epoxy composites with 40 vol% carnauba fiber and, together with the tensile properties, demonstrated for the first time the carnauba fiber reinforcement effect. The SEM micrographs of the fracture surface revealed an evolution in the fracture mechanisms with an increase in the composite volume fraction of carnauba fibers, going from totally brittle neat epoxy (0%) to ductile-brittle for the 40 vol%. It was also possible to identify several active mechanisms. The FTIR analysis showed expected results, with bands referring to molecular vibrations of functional groups belonging to the basic constituents of NLFs, such as cellulose, hemicellulose, and lignin. The FTIR of the composites showed a variation in the relative intensity of the 1737 cm3 band, thus being able to make a relationship with the intensity presented by the band and the concentration of fibers in the composite. The TG analysis of the composites displayed an intermediate behavior on the thermal stability of the composites, when relating the thermal behavior of the neat epoxy resin and the plain carnauba fiber. The composites, on average, showed significant loss of mass above 300 ◦C. The increase in the volume fraction of carnauba fibers in the composite leads to a reduction in the initial degradation temperature and temperature of maximum degradation rate. The DSC curves disclosed an increase in the temperature of the endothermic peak observed in the neat resin, with the increase in the composite volume fraction of carnauba fibers. This is mainly attributed to the moisture release from the carnauba fiber and a possible contribution to the glass transition temperature (Tg) of the epoxy matrix. DMA results confirm the carnauba fiber reinforcement effect associated with an improvement in the storage modulus, as compared to the neat epoxy. Moreover, the Tg obtained from the loss modulus and tan δ peaks, display for the composite an increase (69–99 ◦C) in comparison with that (64 ◦C) for the neat epoxy. According to the mechanical and thermal properties presented by the carnauba fiber epoxy composites, their reinforcement effect and thermal stability above 300 ◦C make them viable substitutes for other epoxy composites, especially those reinforced with glass fiber.

Author Contributions: R.F.P.J. prepared testing specimens, analyzed the data, and wrote the manuscript; L.d.M.N., and A.T.S. prepared testing specimens; J.V.B.M. performed the tests; F.d.C.G.F. analyzed and validated the data; L.F.C.N. and S.N.M. conceived, coordinated, and reviewed the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors wish to thank the support to this investigation by the Brazilian agencies: CAPES and FAPERJ through research funding and UFCA for conducting characterization analyses. Conflicts of Interest: The authors declare no conflict of interest.

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