R p B a b c d a A R R A A K C D C L P 1 d i T t u p u w P d fi i 2 h 0 Industrial Crops and Products 77 (2015) 691–702 Contents lists available at ScienceDirect Industrial Crops and Products jo ur nal home p age: www.elsev ier .com/ locate / indcrop esidual sisal fibers treated by methane cold plasma discharge for otential application in cement based material .N. Barraa,∗, S.F. Santosb, P.V.A. Bergoa, C. Alves Jr. c, K. Ghavamid, H. Savastano Jr. a Universidade de São Paulo (USP), Departamento de Engenharia de Biossistemas, Pirassununga, SP, Brazil Universidade Estadual Paulista (UNESP), Departamento de Materiais e Tecnologia, Guaratinguetá, SP, Brazil Universidade Federal Rural do Semi-Árido (UFERSA), CiTED—Centro Integrado de Inovaç ão Tecnológica do Semiárido, Mossoró, RN, Brazil Pontifícia Universidade Católica do Rio de Janeiro (PUC-RJ), Departamento de Engenharia Civil., Rio de Janeiro, RJ, Brazil r t i c l e i n f o rticle history: eceived 20 May 2014 eceived in revised form 16 June 2015 ccepted 26 July 2015 vailable online 3 October 2015 eywords: apacitance ielectric constant ontact angle ignocelulosic fibers ortland cement a b s t r a c t The use of residual sisal fiber is becoming more frequent as reinforcement element in organic or inorganic matrix due to its low cost, high abundance in some countries and constitutes a renewable material. However, a significant loss in the mechanical performance in long term has been observed in fiber- cement composites after natural aging. These alternative fibers can be utilized in a hybrid fiber-cement in order to decrease the content of traditionally used synthetic fibers. The objective of this work was to evaluate the potential of the methane cold plasma treatment of 10 min duration on structural and physical properties of the residual sisal fibers to mitigate the degradation mechanisms when applied to cementitious matrices. Moisture sensitivity evaluation by capacitance method, dielectric measurements, X-ray diffraction, Fourier transform infrared (FTIR), spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), angle contact and pullout test were carried out in order to follow the effect of the proposed treatment. Besides, mechanical behavior of untreated and treated sisal fibers was evaluated before and after accelerated aging in cementitious solution at 60 ◦C by 72 h. The results obtained in all these tests confirmed the high potential of the methane cold plasma treatment to delay the degradation of the residual sisal fibers in the presence of a Portland cement environment and these fibers present the higher pullout load and shear stress than one untreated. . Introduction In the past decades, related issues to environment and social evelopment have been the challenge of different fields of the ndustry, as well as, the theme of several studies around the world. hus, the construction industry, known for being an emergent sec- or with significant participation in the economy of developed and ndeveloped countries, have to incorporate the sustainability in its roduction process, since the sector has as drawback the massive se of the natural resources (both renewable and non-renewable), aste generation and a high energy consumption (Ortiz et al., 2009; acheco-Torgal and Jalali, 2011). One way to minimize this back- rop is the replacement of the fossil fuel based fibers by vegetable bers as reinforcement in products such as flat or corrugated roof- ng materials, water containers and cladding panels (Tonoli et al., 013). ∗ Corresponding author. E-mail address: bruna.barra@usp.br (B.N. Barra). ttp://dx.doi.org/10.1016/j.indcrop.2015.07.052 926-6690/© 2015 Published by Elsevier B.V. © 2015 Published by Elsevier B.V. 1.1. Sisal fiber Lignocellulosic fibers have reached a remarkable importance as high specific strength materials, for a broad use in the compos- ite materials. In view of the significant advantages provided by the lignocellulosic fibers, mainly due to low-cost production and large availability in nature, there is an increase in the potential use of these fibers in civil engineering (Pereira et al., 2013; Mármol et al., 2013). Moreover, the vegetable fibers come from renewable, have low density, and they are obtained with low energy consump- tion. Additionally, the creation of this new market could benefit the economy of the producing regions that are generally connected to products of low added value, such as cordage and packing indus- try. Tropical countries such as Brazil have an abundance of crops for fiber suppliers, many of which are located in underdeveloped or developing regions. The diversity of plants that can provide fiber generates numerous possibilities for production and application, such as sisal (Agave sisalana), which is easily available and can be produced even under arid climate conditions (Li et al., 2000). Fur- ther, the partial replacement of synthetic fibers (e.g.polypropylene, dx.doi.org/10.1016/j.indcrop.2015.07.052 http://www.sciencedirect.com/science/journal/09266690 http://www.elsevier.com/locate/indcrop http://crossmark.crossref.org/dialog/?doi=10.1016/j.indcrop.2015.07.052&domain=pdf mailto:bruna.barra@usp.br dx.doi.org/10.1016/j.indcrop.2015.07.052 6 ps and Products 77 (2015) 691–702 g e m p 2 s c t 2 l a a ( 2 fi F t i t m t n w fi 2 1 p m M e a 1 fi s t f c f s o t f c i s s a R K a p p t 2 a 9 u t m m i 92 B.N. Barra et al. / Industrial Cro lass and polyvinyl alcohol) by sisal fibers as a reinforcement lement has been of advantage for the fabrication of composite aterials in the last years (Tonoli et al., 2011; Tan et al., 2012). The use of sisal fiber is becoming more frequent, for exam- le, in the toughening of cementitious composites (Gutiérrez et al., 005; Tan et al., 2012). Besides, some residual sisal fibers commonly old to the paper and matting industry, such as wadding, crude or leaned, and also waste brushed or unbrushed, can be utilized for hese purposes (Bledzki and Gassan, 1999; Satyanarayana et al., 007). However, significant losses in the mechanical performance in ong term have been observed in sisal fiber-cement composites fter natural or accelerated aging, due to the degradation mech- nisms of the cellulose fibers in the cementitious environment Toledo Filho et al., 2000; Savastano Jr. et al., 2005, 2009; Silva et al., 011; Melo Filho et al., 2013). The degradation of the vegetable ber is caused by the alkaline environment with pH > 12 (Toledo ilho et al., 2000). Some progressive degradation mechanisms may ake place, such as the destruction of macromolecular chains dur- ng the partial alkaline hydrolysis of the cellulose, which causes heir rupture and the consequent decrease in the degree of poly- erization. This degradation occurs by the easy movement from he pore water towards the surface of the fibers. Another mecha- ism is the gradual filling of the inner cores of the vegetable fibers ith the hydration products leading to the embrittlement of the bers, reducing their mechanical performance (Melo Filho et al., 013; Silva et al., 2011; Pavasars et al., 2003; Scrivener and Young, 997). These mechanisms could affect some important physical roperties of the material reinforced, such as adhesion toughness echanisms and, consequently, mechanical properties (Bentur and indess, 2007). The volume stability of the fiber in a water based nvironment is also crucial in the conservation of the fiber-matrix dhesion Tonoli et al., 2009). .2. Methane cold plasma The deposition of hydrophobic coatings on the surface of the ber counteracts some degradation mechanisms such as the depo- ition of calcium hydroxide crystals in the interior of the fiber and he attack of other ions from cement suspension on the fiber sur- ace. Recently some applications of plasma-based techniques to oating processes have made a significant progress to improve sur- ace characteristics of the fiber materials (Kim et al., 2006). The urface treatment with cold plasma has been applied in the study f adhesion between several kinds of polymers or biopolymers hrough the modification of the surface free energy for efficient unctionalizing (Mahlberg et al., 1998; Novak et al., 2008). The old plasma treatment is a useful technique that utilizes ion- zed gas, at negative pressure, composed by a mixture of neutral pecies (atoms, molecules and free radicals), electrically charged pecies (electrons, positive and negative ions), photons, radicals, nd excited molecules produced by electric discharge (Wielen and agauskas, 2004; Wielen et al., 2006; Gaiolas et al., 2008, 2009; alia et al., 2011; Costa et al., 2006). Although the surface property lterations obtained with cold plasma treatment are very com- lex, they offer an efficient and reliable mechanism to alter surface roperties of materials without affecting the bulk properties of the reated substrate (Carlsson and Ström, 1991; Rolf and Sparavigna, 010). For example, plasma species do not penetrate deeper than bout 100 × 10−10 m from the surface which means that more than 9% of the bulk of a 10 �m thickness polypropylene film remains nchanged (Hua et al., 1997). The most important factor is that he substrate surface properties change significantly after a few inutes of plasma treatment. Plasma treatment of chemithermo- echanical pulp resulted, as in the case of the pulp, in an increment n the quantity of tagged functionalities, which was seen mainly for Fig. 1. Schematic diagram of the methane gas fragmentation of the methane gas produced in the cold plasma system. Adapted from Kado et al.(2003). carboxyl and carbonyl groups (Östenson et al., 2006). This has sig- nificant implications for the lignocellulosic fiber industry (Olaru et al., 2005; Wielen et al., 2005; Kalia et al., 2011; Anwer and Bhuiyan, 2012). Due to the physical process the treatment does not make use of water and chemicals and thus is considered quick and environmentally friendly without generating any contamination. Besides the operating costs are lower than some chemical treat- ments, such as those based on silanes (Shenton and Stevens, 2001; Indarto et al., 2005; Morent et al., 2008; Felekoglu et al., 2009; Navarro et al., 2009). Based on the outcomes of interactions with materials, cold plas- mas can be classified into the broad categories like as plasma polymerization, plasma treatment, and plasma etching (Siow et al., 2006). When cold plasma is generated from a pure organic gas (e.g., methane) or mixed with other gases, a collision occurs between energetic electrons and gas molecules resulting in the formation of a series of reactive fragments, which are recombined to give rise to a solid polymeric material which is deposited on the surface to be treated, or just some functional groups can be grafted on the surface (Kim et al., 2006; Bozaci et al., 2013). This process is known as plasma polymerization. The plasma contains a variety of species including electrons with energies great enough to break molecular bonds from organic gas by collision (Kumar et al., 2010). Fig. 1 shows a schematic diagram of possible fragments produced in the methane cold plasma for non-equilibrium (non- thermal) discharge. The highlighted region indicates the compound most likely to occur. The thickness of the arrows is correlated to the most probable reactions to occur, as well as, dash line arrows are related to lower probability reactions. Although the dehydrogena- tion of C2H6 rapidly produces C2H4 and C2H2, the contribution of that reaction path is small because the composition of C2H6 in the discharge region is very low due to the low CH3 concentrations as highlighted in Fig. 1 (Kado et al., 2003). It is accepted that in non-thermal plasma systems, the formation of free radicals and ion-radicals is the decisive stage for the consecutive transforma- tions of methane (Ghorbanzadeh et al., 2005). 1.3. Lignocellulosic fiber and methane cold plasma Based on many XPS studies, the vegetable fiber surface has a series of functional groups that rise up, mainly, from lignin and extractives (C C, C H, C O, O C O, COOH, COOC), as well as, from ps and c 2 2 T o r s a r m i s h S o m T l h P m fi o a A B.N. Barra et al. / Industrial Cro ellulose and hemicellulose (C O, C OH) (Hua et al., 1997; Sain, 000; Sain and Panthapulakkal, 2006; Sahin, 2007; Popescu et al., 009; Popescu et al., 2011; Tran et al., 2011; Fuentes et al., 2011). he methane cold plasma discharge induces a series of reactions n the fiber surface, including the formation of hydrophobic alkane adicals (CH3, CH2 and CH) as the result of the interactions with the aturated carbon (C C, C H), hydroxyl carbons (C OH), carbonyl nd carboxyl groups. Simultaneous processes of etching, deposition, degradation, ecombination and crosslinking take place on the surface of aterials that are plasma treated (Östenson et al., 2006). These nteractions could be responsible for the polymerization of the fiber urface, because of the deposition of a thin film of carbon with ydrophobic characteristics (Wielen et al., 2005; Kalia et al., 2011). inha and Panigrahi (2009) proposed that there are two major types f reactions possible with cold plasma on fiber surface: (1) surface odification of polymer and (2) polymerizations of the monomers. hus, with methane cold plasma reactions the characteristics of the ignocellulosic fiber surface such as wettability, adhesive bonding, ydrophilic and hydrophobic tendency are improved (Sadova and ankratova, 2009). In this work the potential use of the methane cold plasma treat- ent to mitigate degradation mechanisms of residual sisal fibers in ber-cement were analyzed. Additionally, the mechanical behavior f untreated and treated sisal fibers was assessed before and after ccelerated aging tests. Fig. 2. (a) Schematic representation of the cold plasma reacto dapted from Costa et al. (2006). Products 77 (2015) 691–702 693 2. Materials and methods 2.1. Materials The residual sisal fibers used are waste of the baler twine, donated by the Associaç ão de Desenvolvimento Sustentável da Região Sisaleira (APAEB-Valente), located in Bahia (one of the northeast Brazilian states), which were tested without any previous conditioning or treatment. 2.2. Plasma treatment The residual sisal fibers were treated in cold plasma reactor vac- uum (Fig. 2a), constituted by a cylindrical tube of borosilicate glass with an outer diameter of 180 mm, an outer height of 300 mm, a capacity of about 7.6 L and two stainless steel flanges (Fig. 2b). The cold plasma treatment with methane gas was generated by direct electric current during 10 min with a gas flow of 5 cm3/min, a dis- tance between cathode and support fibers equal to 70 mm, pressure 6.5 × 10−4 MPa; approximately 450 V and electric current of 0.10 A. 2.3. X-ray diffraction measurements For the X-ray diffraction measurements, the residual sisal fibers were cut in small pieces and fixed in a small plate with inert paste, in order to interact with the beam. The X-ray diffraction patterns were r vacuum; (b) details of the borosilicate glass cylinder. 694 B.N. Barra et al. / Industrial Crops and Products 77 (2015) 691–702 e diele o R a T 2 p ( 2 e a a v o a 2 t s i a c d c s a e m p d i 2 l p e t a m u 8 d a m Fig. 3. Schematic drawing representing the capacitance method to evaluate th btained by the X-ray generator theta–theta from diffractometer IGAKU (Japan) Rotaflex model Ru-200B operating at 40 kV, 20 mA, nd with cooper tube, �(Cu K�) = 1.5406 Å, at room temperature. he beam angle (2�) varied from 10 to 30◦. .4. Infrared spectroscopy FTIR spectroscopy is a nondestructive method for studying the hysic-chemical properties of a bulk material. Infrared spectra FTIR) of the fibers were recorded between 4000 and 600 cm−1 at cm−1 of resolution, with a Spectrum One (PerkinElmer) spectrom- ter, supplied with a universal attenuated total reflectance (UATR) ccessory. For each spectrum, 26 scans were co-added. A small mount of fibers was carefully placed over the cell and pressed con- eniently in order to maintain the same pressure, for reflectance peration mode. Measurements were performed at room temper- ture. .5. Moisture adsorption Sorption is most often described as a common term when hinking of the phenomena adsorption (gain of moisture from the urrounding air) and desorption (loss of moisture to the surround- ng air). In this work the moisture gain in sisal fibers was evaluated t different periods, for untreated and treated fibers by methane old plasma discharge (10 min). For this test, the fiber samples were ried in a stove (Marconi, model MA035, Brazil) with forced air cir- ulation at 60 ◦C for a period of 1 h, then weighed in an analytic cale Mod. Marte (USA). After this the samples were conditioned t 98% relative humidity and 25 ± 2 ◦C at different periods until the stablishment of the equilibrium. For periods beyond 8 days, the oisture content was found to attain the equilibrium for all sam- les. The moisture content (X) is expressed always on the basis of ry solids (X = g water/g dry solids). Measurements were performed n duplicate. .6. Capacitance method Dielectric property is the ability of electrical dipoles in an insu- ator to polarize under an external electric field. Materials that ossess electric dipoles including water molecules exhibit consid- rable dielectric properties. Therefore the dielectric constant (and hus, capacitance) of wet and dry materials changes considerably ccording to the amount of moisture adsorbed. For capacitance easurements, a capacimeter mod. Instrutherm CP-400 (USA) was sed. This instrument applies an alternating electric field near 00 Hz on the capacitance probe (metallic plates). Fig. 3 shows etails and dimensions of the setup used in these experiments nd performed at room temperature. The cell (sample holder) was ade with a transparent Lucite box, assembled in an inverted “U” ctric properties of sisal fibers as function of the amount of moisture adsorbed. shape, with the two rigid and parallel metallic (aluminum) plates mounted as shown in Fig. 3, forming a capacitor. Small amounts of fiber samples were carefully compacted inside small trays made of transparent Lucite. This tray was placed inside the Lucite box, in the region between the metallic plates, for measuring the capaci- tance. For relative humidity (RH) control inside the box, water was used, as shown in the figure, in order to simulate an environment with 100% RH. Before measurements were taken, the fibers were dried in a stove (Marconi, model MA035, Brazil) with forced air cir- culation by 60 ◦C for a period of 2 h in order to remove any trace of moisture content. After this step, the capacitance measurements were taken during the exposure of the fibers inside the Lucite box until the establishment of the equilibrium. The response was the capacitance variations during the exposure time of the fibers at this simulated environment. 2.7. Contact angle goniometer The surface energy can be obtained indirectly through the con- tact angle of a pure liquid droplet on the solid substrate by means of the sessile drop method. The static water contact angle (�) after about 10 s of dropping deionized water at 25 ± 2 ◦C was measured by Krüss Easy Drop goniometer (Krüss, GmbH, Hamburg–Germany) equipped with a digital photo analyzer using the sessile drop tech- nique to observe surface characteristics of the residual sisal fiber before and after methane cold plasma treatment. Multiple droplets of 0.22–0.30 �L were deposited in several places throughout on the single fiber to determine possible heterogeneity, and the average of five fibers was used to calculate each �. The analysis of variance (ANOVA) followed by Tukey test were applied to compare the mean results between groups at a significance level of 5%. 2.8. SPM imaging analysis The surface morphology of the residual sisal fiber before and after methane cold plasma discharge was analyzed using a Scan- ning Probe Microscope (SPM) (NT-MDT, Moscow, Russia), Solver Next. Images were acquired in contact mode in air using tetrahe- dral tip model N-type Si cantilever with a reflective side of gold and a radius between 6 nm and 10 nm. Force curves were measured at the nominal resonance frequency of 1 Hz and a spring constant of the cantilever of 0.03 N/m. 2.9. Accelerated aging In order to assess the effect of the methane cold plasma treat- ment in the residual sisal fiber an accelerated aging test was carried out based on methodology proposed by Ramakrishna and Sundararajan (2005). The sisal fibers of 8 cm length (untreated and treated one) were kept immersed in a borosilicate glass boiling B.N. Barra et al. / Industrial Crops and Table 1 Chemical composition of the ordinary Portland cement CP V-ARI.a Compounds SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O L.O.I.b Content (%wt) 20.14 4.94 3.07 62.87 1.39 3.4 0.95 0.15 2.22 c fl o d a ( 6 i b m c fl i c p o 2 m A t m q 2 m m F o a t s l t m a t p a Brazilian Standard NBR 5733 (clinker + gypsum = 100 − 95% by weight; calcium arbonate filler = 0–5%) b L.O.I.: Loss on ignition ask with three short necks. The accelerated aging test was carried ut for 72 h in saturated ordinary Portland cement solution with eionized water at 60 ± 1 ◦C, controlled by a hot plate and temper- ture sensor system. After this time the fibers were kept in a stove Marconi, model MA035, Brazil) with a forced air circulation by 0 ◦C/24 h and the ultimate tensile strength was determined. Dur- ng the period of exposure, the pH of the solution was maintained etween 11 and 12 with the use of a pH meter (DM-23, DME-CV1 odel, Digimed, Brazil). The carbonation of the ordinary Portland ement solution was avoided by sealing the necks of glass boiling ask. The Brazilian ordinary Portland cement, type CP V-ARI (high nitial resistance, Table 1) was utilized in this work. The use of this ement is justified because it is commercially available, without ozzolanic or blast furnace slag additions and has a lower content f limestone filler. .10. Scanning electrons microscope The residual sisal fibers were examined by scanning electron icroscope (SEM) in backscattered electron mode using Hitachi nalytical Table Top Microscope TM 3000 and the environmen- al scanning electron microscope (ESEM) in backscattered electron ode and energy dispersive X-ray spectroscopy (EDS) using FEI uanta 600, before and after accelerated aging test. .11. Mechanical properties The tensile strength of the residual sisal fiber was deter- ined using a single residual sisal fiber in an universal testing achine EMIC DL 30000 with a displacement speed of 0.4 mm/s. ifteen fibers were tested. The transversal section of the fibers was btained by an optical microscopy Zeiss, model AxioImager.A2m nd a digital camera model Axiocam MRc5 (German). According o the observations, it was found that the cross section area of the isal fibers is variable and its geometry is roughly elliptical along the ength. For this reason, a correction factor was determined between he actual area and the elliptical area which is calculated from the ajor and minor axes of the fiber cross section. Besides, the real rea was considered, corrected by a correction factor for each fiber ested (Motta et al., 2010). The fiber was conveniently fixed in the aper mask apparatus, according to the scheme of Fig. 4, in the Fig. 4. Schematic representation of the m Products 77 (2015) 691–702 695 attempt to avoid tension concentration or eccentricity during the development of the test. 2.12. Pullout test For the pullout test cement paste cylinders were molded with a single fiber centrally aligned with an embedded length of 25 mm, in accordance with the methodology used by Ferreira et al. (2012). The matrix composed of filler, binders (30% Portland cement, 30% metakaolin, 40% fly ash) and water. The cylinder specimens remained in the mold for 24 h and after demolding were kept in a humid chamber for 7 days to cure. After curing, the specimens remained for one day under 23 ◦C ± 1 ◦C temperature and relative humidity of 43% ± 3% for removal of excess water present in the fiber. After drying the specimens were tested in electromechani- cal universal testing machine, Shimadzu, AGS-X, with a load cell of 1 kN and displacement rate of 0.1 mm/min. From the pullout test, it was calculated the shear stress, �, between fiber and matrix using the following equation: � = P 2�rL (1) where P is load, L is embedded length of the fiber and r is the fiber radius obtained by scanning electron microscopy. 3. Results and discussions 3.1. SEM analysis Fig. 5 shows the untreated surface of a residual sisal fiber with the deposition of calcium oxalate microplates. Many vegetable fibers accumulate calcium oxalate as it has been reported in dif- ferent vegetable species. The calcium oxalate crystals can occur in a wide range of morphologies. These morphologies include block- like prismatic crystals, present as single or multiple crystals, with large elongated rectangular shapes (Franceschi and Nakata, 2005). They are highly insoluble and dissolve poorly in water. However, in the sisal fiber there is not a large enough quantity of cal- cium oxalates to be detected by the ordinary X-ray diffractometer method. The SEM images of untreated and treated sisal fibers after accel- erated aging are shown in Fig. 6. The cement solution produced some compounds such as SiO2(H2O)x, K+, Na+, SO2− 4 , Al3+ and OH− ions that were highly effective at binding the calcium ions (Taylor, 1997) and thereby decreasing the concentration of calcium in solu- tion, shifting the reaction equilibrium so that more calcium oxalate of the sisal fiber can be dissolved. Although both, untreated and treated, surface sisal fibers had been attacked by ions in the interval of three days in the saturated echanical tests of the sisal fibers. 696 B.N. Barra et al. / Industrial Crops and Products 77 (2015) 691–702 Fig. 5. SEM images in backscattered electron mode of untreated surface sisal fiber with respective detail of calcium oxalate microplate. The number “1” indicates the point of the energy dispersive X-ray spectrum. Fig. 6. SEM images in backscattered electron mode of surface sisal fiber after accelerated aging with respective amplified images showing details of deteriorated calcium oxalate microplates and surfaces. Untreated (a) and (b), and treated sisal fibers (c) and (d). B.N. Barra et al. / Industrial Crops and Products 77 (2015) 691–702 697 c t 3 d c m ( m O t fi T o r t m t i w T c p t o g o u 3 i 2 t a h a t i e s t i n v Fig. 7. X-ray diffraction pattern of untreated and treated sisal fibers. ement solution at 60 ◦C there is a significant difference between heir surface structures. .2. X-ray diffractions patterns The structure of sisal fibers is complex, and in that it comprises ifferent hierarchical microstructures. This is basically due to the omposite-like structure of lignocellulosic fibers that are mainly ade up of a complex network of three biopolymers: cellulose semi crystalline), hemicellulose (amorphous) and the lignin (aro- atic polymer) (Satyanarayana et al., 2007; Moniruzzaman and no, 2013). The X-ray diffraction patterns depicted in Fig. 7 show hree peaks at 2� (degree) equal to 14.76◦, 16.44◦ and 22.56◦ con- rmed that mainly native cellulose is present (Liu and Hu, 2008). hese peaks are also related to the semi crystalline main fraction f the cellulose microfibril. The amorphous fraction is associated to egion from 2� value of 10–20◦. Thus, according to the X-ray diffrac- ion patterns, the sisal fiber bulk was not apparently changed by ethane cold plasma treatment. The fiber is assembled by gluing ogether a bundle of very small fibers called fibril. The glue hold- ng the fibrils together is lignin. Fibrils are made up of microfibrils hich in turn are made up of cellulose and hemicellulose polymers. he cellulose microfibril can be considered as a single thin and long rystalline and amorphous entity with highly anisotropic physical roperties (Nishiyama, 2009). The free hydroxyl groups present in he cellulose macromolecules are likely to be involved in a number f intramolecular and intermolecular hydrogen bonds, which may ive rise to various ordered crystalline arrangements. In the case f cellulose and its derivatives, these crystalline arrangements are sually imperfect (Pérez and Mazeau, 2005). .3. FTIR results The FTIR spectra of untreated and treated sisal fibers are shown n Fig. 8. The dominant peaks in the region between 3600 and 800 cm−1 are due to stretching vibrations of OH and CH, respec- ively. The prominent peaks between 1737 cm−1 in the sisal fiber re attributed to either the acetyl and uronic ester groups of the emicelluloses or the ester linkage of carboxylic group of the ferulic nd p-coumeric acids of lignin and/or hemicelluloses. The absorp- ion at 1650 cm−1 is principally associated with absorbed water n crystalline cellulose (Sinha and Panigrahi, 2009; Kačuráková t al., 1998). The region from 1436 cm−1 to 1385 cm−1 repre- ents the aromatic C C stretch of aromatic rings of lignin and he peaks can reflect C H asymmetric deformations. The peaks n the region 1200–950 cm−1 are due to C O stretching. The peak ear 1017 cm−1 is assigned to strong hydrogen bond or C O H ibrations. The increase of the band at 896 cm−1 indicates the typ- Fig. 8. FTIR spectra of the untreated and treated sisal fibers. ical structure of cellulose (Alemdar and Sain, 2008; Alvarez and Vázquez, 2006). The FTIR technique showed also that there are no significant differences between untreated and treated sisal fibers in the peaks near 2800 cm−1 and 1737 cm−1. At 2800 cm−1 two weak peaks merged and at 1737 cm−1 a new peak appeared after methane cold plasma treatment indicating some chemical modifi- cations. 3.4. Surface topography The surface morphology of the untreated and treated fiber sur- faces derived from SPM in contact mode are shown in Fig. 9. These images detail the rough primary wall and middle lamella which is characteristic of fibers. However, it is difficult to make a distinc- tion between the two layers. The middle lamella is very thin and composed of 70% lignin associated with a small amount of hemicel- lulose, cellulose and pectin, which binds the microfibrils. Therefore, they are often referred to as composed lamella. (Xu, 2010; Gandini and Belgacem, 2008). The treatment affected the surface roughness of the fibers. The root-mean-square surface roughness (measured for 4.0 �m × 4.0 �m) was measured. The untreated fiber presented a root mean square roughness, RMS, of about 59 nm and treated fiber about 37 nm. Kim et al. (2006) have reported that the CH4 plasma polymerization deposited a very smooth hydrocarbon layer exposing CH2 and CH3 groups leading to increased hydropho- bicity. 3.5. Moisture content Fig. 10 shows the moisture content of untreated and treated sisal fibers when exposed to 98% relative humidity and 25 ± 2 ◦C at different periods until the establishment of the equilibrium. The sisal fiber contains about 74–75.2 wt% of �-cellulose, 10–13.9 wt% of hemicellulose and 7.6–8 wt% of lignin (Satyanarayana et al., 2007). The hemicellulose is a branched polysaccharide composed mainly of pentosanes and hexosanes, which contain a large number of hydroxyl groups. Because of its branched formation and com- plex chemical structure, its contribution to the polar behavior is higher than cellulose. Finally, lignin is an aromatic polymer with a high content of branched molecules (Rangel-Vázquez and Leal- García, 2010). The hydrophobic nature of the lignin on the other hand acts as a cementing agent and increases the stiffness of the cellulose/hemicellulose composite. Therefore, the lignin is a water- proof material, enabling the transport of water and solutes through 698 B.N. Barra et al. / Industrial Crops and Products 77 (2015) 691–702 F imag p t p a i g c u m s s c ig. 9. Morphology of fiber surfaces obtained from deflection SPM image data. SPM lasma treatment. he vascular system, and plays a role in protecting plants against athogens (Boerjan et al., 2003; Thomas et al., 2011). Therefore, ccording to the results, the untreated fibers presented more the nherent polar and hygroscopic nature of the celluloses given by the reater amount of water absorbed. In treated fibers by the methane old plasma, the water absorbed amount is smaller than in the ntreated fibers. Thus, the treatment of 10 min provided enough odification in the surface and electrical properties of the residual isal fiber. This behavior was also found in another work, including amples of cellulosic fibers treated by dielectric barrier dis- harge (Wielen et al., 2006). Functional groups and cross-links are Fig. 10. Moisture content of the untreated and treated sisal fibers. es in 2D and 3D, respectively, before (a) and (b) and after (c) and (d) methane cold introduced at the fiber surface by reaction among gas-phase species and surface species. Plasma polymerization involves the fragmentation and subsequent deposition of hydrophobic organic monomers that can delay adsorbed water in the fiber (Siow et al., 2006; Friedrich, 2011). At surfaces of fiber, water can be adsorbed at three levels: monolayer-adsorbed moisture, adsorbed moisture and condensed moisture. Adsorbed water molecules are affected by influencing van der Waals interactions, as well as the ratio of the binding to diffusion forces for water molecules on the solid surface. The ini- tially adsorbed water molecules in high energy sorption centers in the solid surface can form a monomolecular layer. As more water molecules adsorb onto the surface fiber, they are subjected to sur- face binding and the diffusion forces induce water to penetrate into the fiber through the microporous. Simultaneously, condensed moisture like gel is formed onto monolayer. And finally, the water can enter in multilayer and free water domain at the fiber surface and lumens. 3.6. Capacitance measurements Fig. 11 illustrates the dielectric response of the fiber samples subjected to a 100% relative humidity atmosphere. The capaci- tance increased with the increasing of the water adsorption until the establishment of the equilibrium (moisture saturation in the fiber) for both types of fiber (Fig. 11). The time elapsed to untreated B.N. Barra et al. / Industrial Crops and Products 77 (2015) 691–702 699 Fig. 11. The capacitance characteristics of the untreated and treated sisal fibers as a function of water adsorption time. Table 2 Dielectric constant values of sisal fibers before and after treatment with discharge plasma. fi a s t i i t m b A s w b T a s d h f c s c m d m t t 3 b s h o t c c Fig. 12. Images of water droplets on surface fibers with contact angle representa- tions. (a) Untreated and (b) treated fibers. Table 3 Contact angle values of sisal fibers.* Condition Untreated Treated Contact angle (degree) 83◦ ± 13◦a 105◦ ± 4◦b test. The results (Table 4) indicate that the treated fiber presents higher pullout load and shear stress. According to Ferreira et al. (2012) four regions are identified in the curve of the untreated fiber: the first (I) is the linear elastic region in which the adhesion is Table 4 Results of pullout test of sisal fibers. L (mm) Treatment P � P � Condition Untreated Treated Dielectric Constant value 1.5 ± 0.3 3.1 ± 0.3 bers to attain the equilibrium can be estimated to be near 300 min nd for treated fibers near 200 min, which means that the hygro- copic nature of fibers was affected by the methane cold plasma reatment. The variations of the capacitance with water content n cellulosic fiber can be explained by the way the water molecule nteracts with the biopolymer by the chemical and physical adsorp- ions of water molecules existing in the studied atmosphere. As oisture increases, an additional layer of water molecules begin to e formed, over an already formed chemisorbed layer (monolayer). lso it was observed that as moisture increases on the surface of isal fiber a great number of physical adsorption layers are formed, hich in turn markedly affects the measured capacitance, as can e seen by the results depicted in Fig. 11 (Mahadeva et al., 2011). herefore, the hydrophilic groups like C–O and C–OH, were affected fter treatment. The cold plasma using methane gas (CH4) can be causing the urface polymerization, providing the formation of hydrophobic ipolar alkane radicals (CH3, CH2 and CH), which could explain the ighest dielectric constant (Table 2). There is a collective effect of induced dipoles (contributions rom bond water and hydrophobic dipolar alkane radicals) on the apacitance of the treated fiber surface. In molecules located at the urface, the dipole moment results from the fact that there is a spe- ific distribution of positive and negative charges. In fact, the dipole oment of a molecule is determined by integration over the charge istributions on the surface. The deposition of hydrophobic organic onomers (free radicals hydrophobic groups) contributed strongly o increase the dielectric constant in the first stage of hydration of he treated fiber. .7. Contact angle measurements The surface energy of the fiber is closely related to the hydropho- icity, and hence with the contact angle between a liquid and fiber urface. The increase of the contact angle with the water means ydrophobization of the fiber surface. Fig. 12 shows the effect f the methane cold plasma treatment on the surface energy of he residual sisal fibers. The treatment significantly increased the ontact angle of the sisal fiber (Table 3). A higher contact angle indi- ates a higher difficulty in forming a water monolayer on the fiber * Columns with means followed by different letters differ by Tukey test at 5% probability (P < 0.05). surface, as well as adsorption of water is retarded. Thus, this result corroborated to moisture measurements as shown, previously, in Fig. 12. 3.8. Ultimate tensile strength distribution The ultimate tensile strength values of the residual sisal fibers varied between 129 MPa and 378 MPa before accelerated aging, according to the distribution showed in Fig. 13a. These results indicate that the residual fibers are weaker than non-residual sisal mentioned in the literature, where the ultimate tensile strength values vary between 324 MPa and 577 MPa, according to Satyanarayana et al. (2007). Therefore, this significant difference in ultimate tensile strength distribution can be attributed to defects of the residual fiber structure and surface, probably because these fibers are wastes generated during the manufacturing of useful fibers that are used in the industry. Fig. 13b and c shows the frequency distribution of the ultimate tensile strength of the untreated and treated residual sisal fibers after the accelerated aging test by 72 h. Considering the range of tensile strength values found in this work, it was observed that the untreated fibers decreased the ultimate tensile strength distribu- tion to lower than 70 MPa, but the values of the treated residual fibers were maintained between 150 MPa and 400 MPa. Fig. 14a and b indicates the fiber break region after accelerated aging. As can be seen, untreated fibers present the fibrils peeled from the hemicellulose and lignin matrix, because of the degrada- tion caused by the high alkalinity of the Brazilian ordinary cement Portland solution. This fact can be responsible for the low values of ultimate tensile strength. In contrast, treated fibers show the matrix preserved by the methane cold plasma treatment, helping to maintain the ultimate tensile strength of these fibers. 3.9. Pullout results Fig. 15 shows the typical load-slip curves obtained in the pullout max max Slip Slip (N) (MPa) (N) (MPa) 25 Untreated 3.73 (1.02) 0.30 (0.08) 2.56 (0.72) 0.18 (0.04) Treated 5.74 (0.77) 0.44 (0.08) 4.64 (0.72) 0.33 (0.04) 700 B.N. Barra et al. / Industrial Crops and Products 77 (2015) 691–702 Fig. 13. Ultimate tensile strength frequency distribution of the residual sisal fibers. (a) Untreated residual sisal fiber before and (b) after accelerated aging, and (c) treated fiber after accelerated aging. Fig. 14. Fig. 15. (a) Typical curves of the pullout test of treated and untreated sisal fibers; fiber surface (b) untreated and (c) treated after pullout test. ps and c i t m h r fi i p p t t c c 4 s I s i c m t d h 5 fi e l c t • • • • • • • • B.N. Barra et al. / Industrial Cro onsidered perfect. In region II it can be observed the loss of linear- ty which is defined as the starting point of detachment between he fiber and matrix. This point is related to the interfacial detach- ent that propagates stably until the load reaches the peak, the ighest pullout load (Pmax). The region III, after the peak load, cor- esponds to the stage where the detachment is complete, or the ber is entirely detached from the cement matrix. The region IV, ndicated by Pslip, corresponds in fact to the slippage or debonding rocess of the fiber. The results of the pullout tests indicated that plasma treatment romoted a degree of surface modification (Fig. 15b and c) and let he sisal fibers less subjected to the wet/dry shrinkage process; i.e. he treatment improved the interface between the fiber and brittle ement matrix without loss of adhesion by the oscillation of the ross section when fiber is submitted to wetting and drying cycles. . Implications The fibers treated by methane cold plasma during 10 min pre- ented a higher hydrophobicity when compared to untreated fibers. n addition, the treated fibers retained a good residual ultimate ten- ile strength distribution even after a severe accelerated aging test n saturated cement solution at 60 ◦C during 72 h. The results indi- ate clearly the potential of the methane cold plasma treatment to itigate degradation of the fiber structure in cement matrix. These reated sisal fibers have a better adhesion on cement matrix and emonstrated that can be used for practical applications, including ybrid reinforcement in fiber-cement composites. . Conclusions The cold methane plasma treatment induced significant super- cial changes in the residual sisal fibers. The overall trend can be xplained by the possible formation of an external polymerized ayer on the surface, contributing to the increase of the dielectric onstant and reducing the water absorption. From this experimen- al study, the following comments can be addressed: Untreated and treated surface sisal fibers were attacked by ions of the saturated cement Portland solution at 60 ◦C during three days. There was a significant difference between the surfaces observed regarding the calcium oxalate structures identified by SEM images. According to the X-ray diffraction patterns and FTIR measure- ments the bulk residual sisal fibers were not changed significantly by cold methane plasma treatment. The amount of absorbed water of the treated fibers is smaller than in the case of the corresponding untreated ones. The untreated fiber surface presented root mean square rough- ness, RMS, about 59 nm and treated fiber about 37 nm, according to SPM technique. The capacitance increased with increasing the water adsorption until the equilibrium for both types of fiber, but treated fibers presented a major dielectric constant of 3.1 ± 0.3 more than the double of the value corresponding to the untreated fibers. The methane cold plasma treatment increased the contact angle of the residual sisal fiber surface from 83◦ ± 13◦ to 105◦ ± 4◦. After accelerated aging test it was observed that the untreated fibers decreased the ultimate tensile strength distribution to lower than 70 MPa, but the values of the treated residual fibers were maintained between 150 MPa and 400 MPa. The fiber treated present the higher pullout load and shear stress than one untreated. Products 77 (2015) 691–702 701 Acknowledgments The authors acknowledge the Brazilian financial support from Fundaç ão de Amparo à Pesquisa do Estado de São Paulo (FAPESP, Grants nos.: 2008/04769-9, 2009/10614-0, 2009/17293-5; 2010/16524-0), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Grants nos.: 472133/2009-8, 305792/2009-1 and 310259/2010-0), Coordenaç ão de Aperfeiç oamento de Pessoal de Nível Superior (CAPES, Projeto Pro Engenharias—PE 103/2008). The authors are gratefully acknowledged to Saulo R. Ferreira and Flávio A. 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