biodegradable composites

Composites Part B 106 (2016) 88e98

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Composites Part B journal homepage: www.elsevier.com/locate/compositesb

The effect of surface treatment on the performance of flax/ biodegradable composites Panayiotis Georgiopoulos a, Aggelos Christopoulos a, Stefanos Koutsoumpis b, Evagelia Kontou a, * a b

Mechanics Department, National Technical University of Athens, Iroon Polytechniou 9, Zografou, 15780, Athens, Greece Physics Department, National Technical University of Athens, Iroon Polytechniou 9, Zografou, 15780, Athens, Greece

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 July 2016 Received in revised form 31 August 2016 Accepted 8 September 2016 Available online 10 September 2016

In the present study, three series of natural fiber-polymeric composite materials have been prepared and experimentally studied, in terms of Scanning Electron Microscopy, Differential Scanning Calorimetry, Tensile-Flexural testing, Dynamic mechanical analysis and Creep. The materials are unidirectional flax fiber composites, prepared with the “film stacking method”, based on three types of biodegradable polymeric matrices. The addition of flax fibers with this method leads to a significant toughening of the polymers under investigation. In order to further improve the composites thermomechanical performance, three different surface treatments of flax fibers, namely silanization, plasticization and treatment with maleic anhydride, were employed. The effect of the fibers treatment on the mechanical properties of the composites has been comparatively studied and discussed. The surface treatment with maleic anhydride resulted to the highest Young's modulus increment, whereas silanization and plasticization improved both tensile and flexural properties. Similar effect was obtained for storage modulus increment, as well creep resistance of the composites with modified flax fibers. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Polymer-matrix composites (PMCs) Fibres Mechanical properties Rheological properties

1. Introduction In recent years, due to the growing environmental awareness and the necessity for developing sustainable materials [1], there is a trend of replacing conventional fibers (glass, carbon), which are predominantly used as reinforcements in polymer matrices, with bio-fibers, i.e. fibers derived from natural sources such as plants, animals and minerals [2]. Low cost, low specific weight, flexibility in processing, competitive mechanical properties, recyclability, safer handling and working conditions but principally their contribution to the environmental issue, are the advantages of biofibers, in comparison with glass fibers, and the reason for the growing trend of using them [2e5]. However, there are compounding difficulties due to the inherent polar and hydrophilic nature of natural fibers, as a result of the strongly polarized hydroxyl groups of lignocellulose [6], and the non-polar features of most thermoplastics. This results to non-uniform dispersion of the fibers, while their high moisture absorption leads to bad adhesion

* Corresponding author. E-mail address: [email protected] (E. Kontou). http://dx.doi.org/10.1016/j.compositesb.2016.09.027 1359-8368/© 2016 Elsevier Ltd. All rights reserved.

with the matrix with the creation of voids at the interface [2]. Plantbased natural fibers consist of cellulose, hemicellulose, lignin, pectins and waxes. Their mechanical properties, thus, their reinforcing efficiency, depend on the content, nature and crystallinity of cellulose, and the angle of microfibrils [2e7]. Cellulose is a natural polymer, consisting of D-anhydroglucose (C6H11O5) repeating units, joined by 1,4-b-D-glycosidic linkages at C1 and C4 position [8]. Among the various natural fibers, flax, kenaf, jute, hemp, coir, bamboo, ramie and sisal [4,9], are of particular interest. In order to preserve the environmental advantages and the ecoefficiency of the natural fibers, biodegradable or recyclable polymers are considered as an ideal matrix for this kind of composites [10]. Biodegradability is an alternative eco-efficient solution for the end-of-life disposal of polymer products, if recyclability is not an option [11]. The so-called “green composites”, by reinforcing biopolymers with natural fibers [12], due to their significant environmentally beneficial properties, create a new perspective for the composites technology. Construction, decoration, decks, roofs, auto-motive and soundproofing are some of the continuously increasing applications, or fields of application of biocomposites, exploiting their high stiffness and strength [2,3]. By using a biodegradable material as a matrix, the derived bio-composite is

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totally biodegradable, thus, there is a new perspective by producing limited life-time polymer composites, designed to biodegrade after use. This kind of materials may find interesting applications as temporary structural material, mainly in agriculture and construction field [13]. Hydrophilic nature of most of the biodegradable polymers leads to the assumption of better interaction with natural fibers. Specifically for Poly-Lactic Acid (PLA), its polar structure is expected to provide improved adhesion of the fibers, thus, improved properties for the bio-composite [14]. This is not confirmed by Oksman et al. [15], as pull-outs and clean fiber surfaces were noticed in the fracture surface. In recent years, material systems, based on a biodegradable matrix reinforced with flax fibers have been studied [11,13,15e23]. Polylactic acid (PLA) in particular, due to its competitive properties, has been examined as a matrix reinforced with kenaf fibers [24e26], bamboo fibers [27], hemp [28], jute [29,30] etc. The study of biodegradation procedure of a variety of composites, is focused on the special behavior exhibited by each specific matrix [11,25,27]. In a PLA/bamboo fiber composite, it was found that its degradability could be controlled by the amount of the compatibilizing agent, namely LDI (Lysine based diisocyanate). The degree of fibers adhesion on the matrix was effected by the LDI content that has been employed [27]. In a composite material, based on PLA and kenaf fibers, it was found that the biodegradation rate is increasing due to the presence of the fibers [25]. Another interesting topic is the role of plasticizers, employed for the compatibilization of PLA, and their absorption by the natural fibers [15]. Limited studies are reported, to our knowledge, for unidirectional natural fiber composites, regarding mostly polypropylene matrix [31e33] and epoxy resin [34e36]. Regarding biodegradable unidirectional natural fiber composites, studies have been made for PLA composites [24,33] and starch composites [22,37]. A major problem of the effectiveness of natural fiber/polymer composites is the incompatibility between hydrophilic fibers and hydrophobic matrix. This can lead to a poor fiber/matrix adhesion due to the presence of pendant hydroxyl and polar groups in the components. As a consequence, high moisture uptake takes place leading to a deterioration of the mechanical properties [3] Therefore, an important issue in studying polymer/natural fiber composites is the improvement of the fiber-matrix interaction, either by modifying the fiber's surface or by the matrix modification employing additives called coupling agents. Combination of the two approaches has been also reported [38]. In Ref. [39] five different chemical treatments were applied to examine their effect on the composites mechanical performance. Referring to chemical modification, it is meant as a chemical reaction between some constituents of the natural fiber and a chemical reagent, forming a covalent bond between them. This results in both, a better fiber dispersion, which is usually restricted due to the hydrogen bonding between fibers, and bond formation between fiber and polymer matrix [9]. The scope of this study is the evaluation of the effectiveness of three different biodegradable polymers as matrix materials for unidirectional flax fiber composites, prepared with the “film stacking method”, a procedure which has not been studied sufficiently so far. Previous work [40] has been focused on manufacturing unidirectional plant fiber reinforced composites, as well as on the use of a novel plant fiber yarn surface treatment method to enhance mechanical properties and fiber-matrix adhesion. Therefore, orientation of the fibers and less thermomechanical processing, as mixing is avoided, are considered to be the advantages of this method employed for the preparation of composites. In the majority of previous studies on these composites, the reinforced matrix is polypropylene, due to its low cost and thermal stability. The composites prepared in the present study, are essentially novel materials due to both, the matrix material and the

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unidirectional character of the fibers incorporated. In addition, three different methods improving the compatibility between matrix and fiber were applied, namely treatment of flax fibers with silane, plasticizer and maleic anhydride. Only few works have been previously done, to comparatively analyze the effect of similar fiber treatments on the thermomechanical performance on the materials. The thermal and mechanical properties of the prepared composites were comparatively investigated, providing information about the fibers/polymers efficiency to the mechanical enhancement, for each fiber's modification procedure employed. 2. Experimental part 2.1. Materials Natural fiber polymer composites were prepared, based on three different types of polymer biodegradable matrices as follows. Poly-lactic Acid (PLA), which was supplied by NatureWorks LLC. The selected grade 2002D has a D content of 4.25%, a residual monomer content of 0.3% and a density of 1.24 g/cm3. The material in pellets form was dried at 45
C for a minimum of 8 h prior to use in a desiccating dryer. The second matrix under the commercial name Ecovio® was supplied by BASF SE (Ludwigshafen, Germany). The selected grade Ecovio® L BX 8145 (EC), is basically a blend of poly(butylene adipate-terephthalate) copolyester (Ecoflex® F BX 7011), which is based on non-renewable resources, and PLA (NatureWorks). Because of the PLA content, Ecovio® L BX 8145 consists of 45% of renewable resources. The material in pellets form was dried at 75
C for a minimum of 4 h prior to use in a desiccating dryer. The third matrix employed has the commercial name Bionolle® 1001, and is based on Poly-butylene succinate, of an average density of 1.26 g/cm3, kindly provided by Flexopack S.A. (Athens, Greece). Very common technologies for natural fiber composite materials are resin transfer molding, vacuum injection molding, structural reacting injection molding, injection molding, and compression molding. Regarding PLA-natural fiber composites, their preparation usually involves a mixing of PLA matrix and fibers, after specific conditions of drying, and a hot pressing procedure [21]. Another type of manufacturing is the melt mixing procedure using a twin-screw extruder, or a solution mixing procedure, followed by a compression molding process hereafter, with the processing conditions being a subject of research [19,25]. In our work, unlike the most published works so far, the production procedure for natural fiber composites adopted, is quite similar to those for the production of conventional unidirectional laminate fiber composites. The three types of polymeric matrices examined, were prepared into thin films of an average thickness of 0.2 mm, by a hot press treatment. Hereafter, following the wellknown film stacking method [16], alternating layers of polymeric film and flax fibers-tissue were placed and hot pressed for 6 h, at a pressure of 5 MPa, and at a temperature equal to 100
C for Bionolle and 124
C for Ecovio® and PLA. According to the followed procedure, the fiber breakage due to shear stresses imposed during the melt-mixing procedure, could be avoided, while an optimization of the fiber orientation could be achieved. By this method, plates of unidirectional fiber polymer composites were prepared, at the same average weight fraction 22% and an average thickness of 1 mm. The corresponding volume fraction was calculated to be equal to 19.5%. The prepared composites are designated as BIONflax, ECflax and PLAflax, based on the three matrices, Bionolle®, Ecovio® and PLA, correspondingly. In addition, three different methods improving the compatibility of flax fibers with the matrix were employed. These methods are: A) silanization, where the flax fibers are immersed in a 2 wt% Vinyl-Triethoxy-

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Silane (VTES) solution in distilled water for 24 h. Hereafter, fibers were washed with distilled water and dried at 60
C overnight [41]. It is assumed that by this procedure, the provided hydrocarbon chains influence the wettability of the fibers and improve their chemical affinity to the polymer matrix. The effect of moisture on the materials properties is thus minimized [9]. B) Plasticization of the fibers. After drying at 100
C for 2 h, a weighed amount of fibers added to a neat plasticizer, namely tributyl citrate (TBC) and the mixture heated at 120
C for 8 h. Hereafter, the fibers were cooling at room temperature, washed with acetone to remove the plasticizer, and dried in a vacuum at room temperature. The amount of plasticizer present was determined by the weight difference [21]. The plasticizer is expected to have two functions, i.e. to overcome weakening of fibers as a result of the moisture lost and increase the ductility of the matrix. C) Treatment with maleic anhydride (MA). The maleic anhydride (MA) was applied in a 10 wt% with respect to the fiber weight. The flax fibers were esterified for 25 h with MA, dissolved in acetone at 50
C, in a solution fiber/solvent ratio equal to 1:25 wt/volume. Hereafter the flax fibers were washed in cold acetone and distilled water [42]. The nomenclature of the flax composites, produced as above, is as follows: MATRIXflax (no treatment), MATRIXflax-Silane, MATRIXflax-TBC, MATRIXflax-MA for each treatment correspondingly. 2.2. SEM analysis Scanning Electron Microscopy (SEM) was performed on a NanoSEM 230 (FEI) using an EverharteThornley Detector (ETD). In order to secure conductivity of the surface for clear imaging, gold sputtering was applied with a nominal thickness of 7 nm using an EMS 550X Sputter Coater. 2.3. DSC experiments Calorimetric measurements (DSC) were carried out using a Setaram DSC 141 instrument, calibrated with an Indium standard. Each sample was heated at a constant heating rate of 5
C/min from 20 up to 180
C, and the thermogram was recorded. DSC samples were taken from the centre and the edge of a 2 mm thick material sheet. Three specimens of each sample were measured and the results were averaged and summarized in Table 1. All composites and the polymeric matrices were subjected to the same heating rate. Table 1 DSC results. Material

Tg (
C)

BION BIONflax BIONflax/Silane BIONflax/TBC BIONflax/MA

Tm (
C)

Tcc (
C)

Heat of fusion DHa (J/g)

Crystallinity change (%)

114 114 116.4 114 115

e e e e e

64.10 67.20 61.96 54.86 64.30

e 4.70 3.30 14.4 e

EC ECflax ECflax/Silane ECflax/TBC ECflax/MA

60 60 58 60 57

146/151 146/151 146/153 139/147 146/153

110 107 106 e 106

4.7 5.8 1.51 13.0 8.27

e 23.4 67 176 76.0

PLA PLAflax PLAflax/Silane PLAflax/TBC PLAflax/MA

58.5 62.0 62.0 58.0 62.0

149/155 149/155 147/157 141/151 149/156

e e e e e

19.5 31.5 25.23 30.86 36.0

e 61.5 29.3 58.2 84.6

a

Heat of fusion normalized to pure polymer mass.

2.4. Tensile testing Tensile measurements were performed with an Instron 1121 type tester, at room temperature. The pure matrices were tested in the form of dumbbell type specimens, of a gauge length of 20 mm, and 3 mm wide and the applied crosshead speed was 0.5 mm/min. This value corresponds to an effective strain rate of 4.16  104 s1. Regarding the composites, square tabs made from fiberglass were bonded to the each end of specimens in order to perform tensile stress experiments. For this type of loading, the primary purpose of the tabs is to protect the composite material from damage by the grips that clamp the specimen ends to apply the axial tensile load. This axial load is introduced into the composite via shear, due to the frictional forces developed between the grip faces and the surfaces of the tabs. The tensile specimens were of 50 mm in gage length and 6 mm in width. The deformation could be measured very accurately with an experimental procedure, which is based on a noncontact method with a laserextensometer, described in detail in a previous work [43]. Five specimens of each material type were tested, and the average scatter is presented in Table 2. In addition, tensile testing of flax fibers has been performed, and the Young's modulus, tensile strength and elongation at break were found equal to 85 GPa, 700 MPa and 1.6% correspondingly. These elastic properties are within the values reported in the literature for flax fibers [9]. 2.5. Flexural testing This mode of deformation is very close to the end-applications of the fiber composites and therefore very important for their design and optimization. A three-point bending procedure was applied on the materials studied, and the load-displacement curve was obtained. The specimens' dimensions were 1 mm in thickness and 12.7 in width, while the distance between the supports was 30 mm. The cross-head speed applied was 0.5 mm/ min. 2.6. Dynamic mechanical analysis (DMA) Dynamic mechanical analysis experiments were performed using the TA Instruments DMA Q800 instrument. The mode of deformation applied was the single cantilever beam, and the mean

Table 2 Tensile and flexural results. Material

Tensile strength (MPa)

Strain at break

Young's modulus (GPa)

Flexural strength (MPa)

BION BIONflax BIONflax/silane BIONflax/TBC BIONflax/MA

35.0 ± 1.8 119.0 ± 6.6 114.0 ± 6.7 113.5 ± 5.6 55.0 ± 2.7

0.3 0.018 0.015 0.015 0.005

0.5 ± 0.055 12.0 ± 0.72 9.70 ± 0.5 13.22 ± 0.83 14.4 ± 0.82

47 ± 1.6 118 ± 6.9 116 ± 5.4 91 ± 4.3 64 ± 3.8

EC ECflax ECflax/silane ECflax/TBC ECflax/MA

18.0 95.0 95.5 68.5 50.0

± ± ± ± ±

0.2 0.015 0.013 0.010 0.005

0.4 ± 0.06 8.0 ± 0.6 13.5 ± 0.69 10.0 ± 0.56 25.0 ± 1.5

25 90 82 74 67

PLA PLAflax PLAflax/silane PLAflax/TBC PLAflax/MA

55.0 ± 2.9 99.0 ± 4.7 102.5 ± 5.2 63.0 ± 4.1 35.0 ± 1.9

0.07 0.016 0.015 0.010 0.003

3.5 ± 0.40 16.0 ± 0.80 12.5 ± 0.90 12.7 ± 0.78 25.0 ± 1.4

110 ± 6.0 140 ± 6.9 117 ± 5.7 83 ± 4.1 63 ± 3.1

1.0 5.1 5.2 4.1 1.8

± ± ± ± ±

1.2 4.8 4.9 4.5 4.0

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dimensions of sample plaques were 12.6 mm in width and 17.5 mm in length. The temperature range varied from 20 up to 180
C. The temperature dependent behavior was studied by monitoring changes in force and phase angle, keeping the amplitude of oscillation constant, and the strain value was 0.1%. The experiments were performed at a frequency value of 1 Hz, and the heating rate was 3
C/min. The storage and loss modulus curves versus temperature were then evaluated. 2.7. Creep experiments Creep experiments have also been performed with TA Q800, at a single cantilever beam mode of deformation, for a specific time period equal to 30 min, at various temperatures varying from 37 up to 180
C, with an interval of 5
C. A constant stress level of 20 MPa has been applied for all material types, and applying the timetemperature superposition (TTS) principle, the creep strain curves were evaluated, for all materials examined. 2.8. Water uptake experiments Water absorption was determined according to ASTM D570 method. After drying for 24 h, the weight and the thickness of the samples were measured. Then, they were immersed in distilled water at room temperature. Each sample was removed from the water, dried by wiping with blotting paper and weighed immediately in order to calculate the water uptake. After each measurement, the samples were immersed in the water again.

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3. Results and discussion 3.1. SEM fractography analysis In Figs. 1e3 representative SEM images of fracture surfaces, as obtained after tensile testing, are depicted for the flax composites examined. In all pictures the unidirectional orientation of flax fibers is confirmed. In Fig. 1a, it can be seen that most of the fibers are well bonded to the BION matrix and those which are broken seem to quite short, having matrix material on their surfaces. In Fig. 1b, BIONflax/MA fracture surface is shown, with a number of fibers covered with substantial amount of polymeric matrix. Furthermore, a number of fibers appear to have a good adhesion with the matrix, and a lower number of pulled out fibers is detected. ECflax/Silane image is presented in Fig. 2a. Most of the fibers are well adhered on the EC matrix, whereas no holes revealing poor adhesion are shown. In Fig. 2b, ECflax/MA composite's image is presented. The majority of the fibers are well connected with the matrix, and are surrounded by polymeric material. The quality of fiber-PLA matrix connection is shown in Fig. 3a, whereas flax fibers are covered with PLA matrix and no pull out effect is observed. Compared with the PLAflax/TBC image in Fig. 3b, a nonhomogeneous fiber distribution is observed, but the fiber-matrix adhesion seems to be satisfactory. 3.2. DSC results The DSC results are summarized in Table 1. For the composites with untreated flax fibers, it is obtained that the glass transition

Fig. 1. SEM pictures of fractured surfaces of: (a) BIONflax, (b) BIONflax/MA.

Fig. 2. SEM pictures of fractured surfaces of: (a) ECflax/Silane, (b) ECflax/MA.

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Fig. 3. SEM pictures of fractured surfaces of: (a) PLAflax, (b) PLAflax/TBC.

temperature Tg and the melting point Tm of the materials is not affected by the flax fibers presence, with the exception of the Tg of Ecflax. The Tg of BION and BION/composites is lower than ambient temperature and not presented in this table. On the other hand, the heat of fusion DHm is increased due to fibers, namely 5, 23 and 61% for BION, EC and PLA correspondingly. This result is an evidence that the flax fibers act as nucleation regions and enhance crystallinity formation into the bulk polymer. During heating, EC and ECflax exhibit a cold-crystallization temperature (Tcc), not shown by the other material types. When cold-crystallization occurs, less perfect crystals are formed, which are driven to melting at higher temperatures. When flax fibers are subjected to surface treatment, as described in the previous paragraphs, the following results are obtained: Glass transition temperature generally is not affected, with the exception of PLAflax/TBC, where Tg is lower than that of pure matrix. The same effect is observed for the melting temperature Tm, which exhibits about 8 to 4
lowering for PLAflax/TBC, due to the plasticization occurred by the presence of TBC. Regarding melting region, the composites based on PLA and EC exhibit two melting points, (Tm1 and Tm2) between 141 and 157
C, for each type of flax fibers treatment. This effect was previously obtained for composite materials based on EC [44]. The first melting peak Tm1 is constant (146
C) for all ECflax composites, with the exception of ECflax/TBC which is 139
C, due to the presence of plasticizer. Tm1 is related to the fusion of imperfect crystallites, formed during cold crystallization upon heating, and Tm2 related to the fusion of new crystallites formed through the melt-recrystallization process [44]. Following Table 1, EC and ECflax composites exhibit a cold crystallization region Tcc around 106
C, characterized by a cold crystallization enthalpy DHcc. The lowering of Tcc for all ECflax composites with respect to pure EC, denotes that flax fibers promote kinetics and extend crystallization on heating [44]. The overall heat of fusion DH, which is the difference (DHm  DHcc), normalized to the weight of pure polymeric matrix, is shown in Table 1. From these values, the percentage crystallinity change for all composites studied, was evaluated and presented in this table. The highest crystallinity increment (about 176%) is obtained for ECflax/TBC, followed by an 84% increment for PLAflax/MA, and a 61% increment for PLAflax composites. This trend is not observed in the composites based on BION matrix.

Fig. 4. Tensile Young's modulus of all flax fiber composites studied.

3.3. Tensile and flexural results The tensile and flexural properties of the matrices and the composites, are summarized in Table 2, and Figs. 4e7. From these

Fig. 5. Tensile strength of all flax fiber composites studied.

P. Georgiopoulos et al. / Composites Part B 106 (2016) 88e98

Fig. 6. Flexural modulus of all flax fiber composites studied.

Fig. 7. Flexural strength of all flax fiber composites studied.

results it is revealed that regarding Young's modulus, and flax fiber composites with no fiber treatment, the highest increment is obtained for BIONflax, while substantial increment is also obtained for ECflax and PLAflax. This degree of enhancement is comparable to that of conventional polymer-glass/carbon reinforced composites, where the fiber volume fraction is usually three times higher than that in the present work. Similar behavior has been reported in Ref. [22], where Mater-Bi® as a matrix was reinforced by unidirectional flax fibers. Among the three types of composites with no fiber treatment examined in the present study, PLAflax attains the highest stiffness (16 GPa), but not the highest percentage increment, while its tensile strength is lower than that of BIONflax. Given that PLA matrix has better elastic properties than Ecovio® and Bionolle®, it is extracted that the PLA matrix-flax fiber adhesion is not adequate, since the tensile strength is strongly affected by the quality of interphase between fiber and matrix [45]. However, compared to previous studies [15] where the natural fibers are randomly dispersed in the PLA matrix, a higher mechanical enhancement of PLA with unidirectional flax fibers, is obtained in

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the present work. After the fibers treatment, the highest percentage increment of Young's modulus is exhibited by ECflax composites, with the ECflax/MA materials showing a modulus over 60 times higher than that of the matrix. In a similar way, for BIONflax and PLAflax composites the highest modulus is observed after the treatment with maleic anhydride, revealing this way that MA improves the stiffness of the composites, whereas this effect is more intense for PLA and EC composites. On the other hand, strain at break is reduced for all flax-fiber composites, due to the presence of fibers, which make the materials more elastic, whereas the most dramatic lowering is obtained for the MA-treated materials. This is in accordance with the lowest tensile strength exhibited by these composites. Young's modulus is measured at low strains, when stress transfer from the matrix to the fibers through the interface is less critical than in the subsequent stages in the nonlinear regime of the stress-strain curve, where final breakage takes place [38]. Therefore, the improved stiffness is not always an indication of a better fiber-matrix adhesion, if the material is driven to failure at a low stress. Combining the tensile results, it can be extracted that the highest enhancement is obtained for BIONflax, followed by BIONflax/TBC, ECflax/Silane, PLAflax and PLAflax/Silane. The various polymeric matrices employed, seem to collaborate in a different way with the flax fibers (treated or no-treated). These remarks appear to be in accordance with the above discussed SEM images. The flexural strength of the fiber composites in comparison to the one of the pure matrix is shown in Table 2. For untreated fibers, a high improvement is obtained for the bending strength of flax composites, compared to the matrice's one. Again, PLAflax attains the highest (nominal) bending strength, but the percentage increment is of the order of 27%, while ECflax and BIONflax appear to have an improvement of the order of 260 and 150% correspondingly. BIONflax, BIONflax/Silane appear to have almost the same flexural strength, and the same is observed for ECflax and ECflax/ silane, whereas PLAflax exhibits the highest flexural strength. In conclusion, with the exception of silane, the fiber-treatment was found to have a detrimental effect on the flexural strength of the composites. As it is mentioned in Refs. [13e15], the differences in chemistry between fibers and matrix play an important role to the mechanical performance of the composites. The use of a compatibilizer should improve the adhesion quality, but it is not known what kind of damage should cause to the matrix material. It has been reported that for BION matrix [13,46] and PLA matrix [15] the tensile strength could not be improved. This effect may be attributed to the mixing procedure followed, due to which the fibers are short and not oriented as in the present study. Regarding PLA, its inherent brittleness prevents the adhesion of the natural fiber. This fact is inversed when a plasticizer is employed in the mixing procedure [15], while the interpretation of silane effectiveness on stiffness improvement is not so clear [38]. Regarding unidirectional natural fiber composites, there is no adequate information so far about the fiber modification on the composite's mechanical performance. In order to evaluate the different reinforcing mechanisms in the composites studied, a set of well known semi-empirical equations developed by Halpin and Tsai [47] for the Young's modulus were employed, such as the rule of mixture for a stress imposed parallel to the fibers direction (parallel spring model):

  Ec ¼ Vf Ef þ 1  Vf Em

(1)

Where Ec, Ef and Em are the Young's moduli of the composite, the fiber and the matrix correspondingly and Vf is the fiber volume fraction. Eq. (1) is the upper bound of a composite's modulus.

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An improved version of Eq. (1) is given by the modified expression [48,49]:

  Ec ¼ kVf Ef þ 1  Vf Em

(2)

Where k is a parameter related to the quality of fiber-matrix adhesion. The general form of the longitudinal modulus, developed by the Halpin eTsai equation is given by:

 Ec ¼ Em

With h ¼

1 þ zhVf

 (3)

1  hVf Ef Em Ef Em

1

(4)

þz

Where z is a measure of reinforcement geometry, which depends upon loading conditions and geometries of the inclusion, that is, the aspect ratio in the case of fiber [47] and the limiting values of z are zero and Infinity. When parameter z takes the value of zero as a lower bound, Eq. (3) gives:

1 V Vm ¼ fþ Ec Ef Em

(5)

Where Vm is the matrix volume fraction and Eq. (5) represents the series connected model, which is the lower bound of a composite modulus. When z tends to infinity, the Halpin eTsai Eq. (3) reduces to Eq. (1). In Table 3 the experimental data of Young's modulus are presented in comparison with the rule of mixtures (Eq. (1)), and the Halpin-Tsai (Eq. (3)). Eq. (3) does not work for the composites which exhibit higher elastic stiffness enhancement than that of the upper bound (Eq. (1)), namely ECflax/MA and PLAflax/MA composites. From Table 3, as it was expected, the experimental data have a deviation from the rule of mixtures, given that this equation assumes perfect adhesion between fibers and matrix and neglects the interactions due to the difference in their Poisson ratios. The lowest deviation between experiment and rule of mixtures is exhibited by the BIONflax/MA, ECflaxSilane and PLAflax composites. The values for PLAflax/MA and ECflax/MA are not included, given that the Young's modulus of these materials is higher than that by the rule of mixtures. Regarding Eq. (3), it is observed that the highest parameter z and the lowest parameter h values are

referring to the BIONflax/MA, ECflax/Silane and PLAflax composites, which denotes that the fiber effectiveness is improved for these material types, and the physical meaning of parameter z is that it is regarded as a reinforcement measure. This result is in accordance with the tensile and flexural experimental data discussed in the above paragraph. 3.4. DMA results From the storage modulus variation versus temperature (Fig. 8), the mechanical reinforcement, observed in tensile testing, is obvious for all fiber composites studied. For the PLAflax, the transition region at about the same temperature domain 60e75
C with PLA matrix, appears to be smoother, and the material flows above 125
C. The storage of the PLA matrix at high temperatures exhibits some increment at the region of 100
C. This effect is attributed to the cold crystallization temperature, that PLA appears to have at the same temperature and the consequent crystallinity increment of PLA matrix [50]. In an earlier work [15], an analogous step at the storage modulus curve of PLAflax at 80
C was found, which was attributed to the cold crystallization temperature of PLAflax. In the present work such an effect has not been obtained, however the PLAflax remains mechanically active up to 130
C. This is due to the crystallinity increment of PLAflax of the order of 62%, as extracted by the DSC results. The transition region of the composite is slightly shifted to lower temperature range, and hereafter, a “leathery” plateau is observed between 80 and 100
C. Similar behavior is shown for ECflax ECflax/Silane and ECflax/MA (Fig. 8). This behavior can be attributed to both, the increased crystallinity of ECflax composites (with the exception of ECflax/Silane), and the very good adhesion established between matrix and fibers and the consequent ability of load transfer from matrix to fibers, as it is also found from Young's modulus increment for ECflax. These two reasons improve the materials stiffness and stability. In addition, this strengthening effect could be connected with the cold-crystallization of ECflax at about 106
C obtained in the DSC experiments. For both composites, PLAflax and ECflax it can be summarized that they are mechanically active, due to the flax fibers, almost above 100
C, while both unreinforced matrices are getting weak above 80
C. The storage modulus of ECflax/TBC composite exhibits a gradually decreasing trend, with no transition apparent, probably to the plasticization effect of TBC. As far as BION matrix is concerned, with its Tg below ambient temperature, the BIONflax materials are more stable at the temperature region examined, and the flow region of the composite

Table 3 Model parameters for the Young's modulus of the composites. Material

Young's modulus (exp) (GPa)

Young's modulus by Eq. (1) (GPa)

Halpin-Tsai Parameters

z

h

BIONflax BIONflax/silane BIONflax/TBC BIONflax/MA

12 9.70 13.2 14.4

17.4 17.4 17.4 17.4

286 160 410 625

0.37 0.51 0.29 0.21

ECflax ECflax/silane ECflax/TBC ECflax/MA

8 13.5 10.0 25.0

17.3 17.3 17.3 17.3

137 579 221 e

0.60 0.27 0.48 e

PLAflax PLAflax/silane PLAflax/TBC PLAflax/MA

16.0 12.5 12.7 25.0

19.3 19.3 19.3 19.3

60 22 23 e

0.27 0.50 0.49 e

P. Georgiopoulos et al. / Composites Part B 106 (2016) 88e98

95

Fig. 8. Storage modulus of all flax fiber composites studied: (a) PLAflax composites, (b) ECflax composites, (c) BIONflax composites.

occurs at about 105
C. This temperature range is quite lower than that for the PLAflax and ECflax, and this is due to the low glass transition temperature of the Bionolle matrix. By summarizing the storage modulus/temperature variation of the three composites, it is revealed that best improvement was achieved for PLAflax composite. Regarding the effect of fiber treatment on the storage modulus variation, an improved enhancement of the maleated and silane treated flax fibers is obvious, for all material types. EC and PLA fiber composites appear to have two transition regions, one in the range of 70e80
C and the other at 130
C. In general, the flax fibers pretreatment, causes a surface modification increasing the surface energy of the fibers, provides better wettability, and thus enhances the polymer-fiber adhesion, due to chemical bonding formation with the matrix [41]. The storage modulus increment with fiber treatment is a stronger evidence of mechanical enhancement, given that in tensile experiments, stiffness can be improved, even without good interfacial adhesion. This is because modulus is measured at low strains, where stress transfer is less crucial. 3.5. Creep results Creep strain of all materials studied has been evaluated for a time and temperature range, not presented here. Hereafter, the master curves of the creep strain, obtained by applying the timetemperature superposition (TTS) principle in the creep-

temperature experiments at a reference temperature of 37
C, were constructed and depicted in Figs. 9e11, exhibiting a good overlap of the isothermal curves. For reasons of clarity, the creep strain experimental data of the pure polymers are not presented here, due to the high strain difference between matrices and composites. A substantial decrease of the creep strain with fibers is obtained, meaning that creep resistance was enhanced by the addition of fillers, and revealing the solid-like behavior of the composites. By comparing the master curves of the three types of untreated fiber composites, the best creep resistance (lower creep strain) is exhibited by BIONflax, followed by PLAflax, while ECflax appears an incremental trend of creep strain at longer times. This may be related to an inadequate stress transfer from the matrix to the fibers in the case of ECflax. In Fig. 9 the creep strain data of BION/flax composites are illustrated, where the BIONflax/MA composite exhibiting the higher creep resistance, reveals a more elastic material, in accordance with the mechanical improvement, shown in the previous paragraphs. The BIONflax, BIONflax/Silane and BIONflax/TBC composites show a similar creep response, with no essential differences. In Figs. 10 and 11, for the ECflax and PLAflax composites a quite similar trend is obtained, with the ECflax/MA being the more solid-like material having the lowest creep strain value. 3.6. Water uptake results The water absorption WA was calculated by Eq. (6):

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WAð%Þ ¼

MðtÞ  M0  100% M0

(6)

where M(t) is the sample weight at time t, and M0 the initial weight. Figs. 12 and 13 show the variation of the water absorbance as a function of square root of immersion time for Bionflax and PLAflax composites representatively. Water content increased with time due to the high hygroscopicity of flax fibers. A rapid water absorbance during the first hours of immersion is observed, until a stage where a plateau (or very lower increment rate) is reached, revealing a Fickian behavior [44]. However, not all samples have reached a saturated moisture level. The percentage water uptake is higher for PLAflax composites (Fig. 13), compared to that of Bionflax (Fig. 12) composites. Regarding the effect of flax fibers treatment on the water uptake, the trend is that the fiber-treated composites

Fig. 9. Creep master curve at a reference temperature of 37 composites.


C

for BIONflax

Fig. 12. Water uptake as a function of square root of immersion time for BIONflax composites. Fig. 10. Creep master curve at a reference temperature of 37
C for ECflax composites.

Fig. 11. Creep master curve at a reference temperature of 37
C for PLAflax composites.

Fig. 13. Water uptake as a function of square root of immersion time for PLAflax composites.

P. Georgiopoulos et al. / Composites Part B 106 (2016) 88e98

exhibit a lower water content. In general, strong intermolecular fiber-matrix bonding decreases the rate of water absorbance of the material [41]. For improved adhesion between matrix and fibers, the rate of water uptake is expected to decrease since there are fewer gaps in the interfacial region, while more hydrophilic groups may be blocked by the coupling effect [51,52]. For PLA/flax composites, after the initial rapid water absorbance, the slope of water uptake is lower for PLAflax/TBC and PLAflax/MA, whereas it is almost the same for PLAflax and PLAflax/Silane. Also, in absolute values, the water uptake is lower for the PLAflax/TBC and PLAflax/ MA treated samples, compared to PLAflax and PLAflax/Silane. This effect reveals a better fiber-matrix adhesion for maleated and plasticized flax fibers. 4. Conclusions The evaluation of the effectiveness of three different biodegradable polymers as matrix materials for unidirectional flax fiber composites, prepared with the “film stacking method” has been extensively studied. Due to the hydrophilic nature of flax fibers and the hydrophobic one of the polymeric structure, poor fiberematrix adhesion may occur, reducing the mechanical enhancement of the composites. Therefore, three different chemical treatments of flax fibers, namely silanization, plasticization and treatment with maleic anhydride were applied, and the produced flax fiber composites were investigated. A significant mechanical enhancement was obtained with the presence of flax fibers, while hereafter, depending on the surface modification type, Young's modulus, flexural modulus, as well as storage modulus was found to be further increased. These results were supported by the SEM photos, where good adhesion between fibers and matrix has been detected, related with a lower number of pulled out fibers. Another evidence for the mechanical enhancement is the crystallinity increment observed in some material types. Regarding Young's modulus, its values varied between 8 GPa for ECflax up to 25 GPa for ECflax/MA and PLAflax/MA, whereas the highest tensile strength was observed in the untreated flax fiber composites, followed by the Silane and TBC treated ones. A similar response was obtained by the flexural experiments. In addition, following the creep strain data, it can be concluded that all the flax composites, especially these with surface treated flax fibers, appear to have a substantial resistance to creep, and therefore reveal as promising materials for load-bearing applications at long time periods. Regarding the water uptake, the highest moisture levels were exhibited by the PLAflax composites, whereas the maleated and plasticized flax fiber/composites had the lowest percentage water content, revealing this way an improved fiber-matrix adhesion. References [1] Yan L, Chouw N, Yuan X. Improving the mechanical properties of natural fibre fabric reinforced epoxy composites by alkali treatment. J Reinf Plast Compos 2012;31(6):425e37. [2] John MJ, Thomas S. Biofibres and biocomposites. Carbohydr Polym 2008;71(3):343e64. [3] Yan L, Chouw N, Jayaraman K. Flax fibre and its composites - a review. Compos B 2014;56:296e317. [4] Faruk O, Bledzki AK, Fink HP, Sain M. Biocomposites reinforced with natural fibers: 2000e2010. Prog Polym Sci 2012;37(11):1552e96. [5] Joshi SV, Drzal LT, Mohanty AK, Arora S. Are natural fiber composites environmentally superior to glass fiber reinforced composites? Compos A 2004;35(3):371e6. [6] Kalia S, Kaith BS, Kaur I. Pretreatments of natural fibers and their application as reinforcing material in polymer composites e a review. Polym Eng Sci 2009;49(7):1253e72. [7] Sanadi AR, Prasad SV, Rohatgi PK. Sunhemp fibre-reinforced polyester. J Mater Sci 1986;21(12):4299e304. [8] Nevell TP, Zeronian SH. Cellulose chemistry and its applications. New York:

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