Influence of sodium bicarbonate treatment on the aging resistance of natural fiber reinforced polymer composites under marine environment

Polymer Testing 80 (2019) 106100

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Material Behaviour

Influence of sodium bicarbonate treatment on the aging resistance of natural fiber reinforced polymer composites under marine environment

T

V. Fiorea,∗, C. Sanfilippoa, L. Calabreseb a b

Department of Engineering, University of Palermo, Viale delle Scienze, Edificio 6, 90128, Palermo, Italy Department of Engineering, University of Messina, Contrada Di Dio (Sant’Agata), 98166, Messina, Italy

ARTICLE INFO

ABSTRACT

Keywords: Sodium bicarbonate treatment Flax Jute Green composites Salt-fog exposition Marine environment

Aim of the current study is to investigate how an innovative and eco-friendly chemical treatment based on sodium bicarbonate solution (10 wt%) can improve the aging resistance in marine environment of epoxy based composites, reinforced with flax and jute fibers. To this scope, treated and untreated fiber reinforced composites were manufactured through vacuum infusion technique. The resulting composites were then exposed to salt-fog spray conditions up to 60 days, according to ASTM B117 standard. The assessment of their durability was made by means of tensile, flexural quasi-static tests and Charpy impact tests. Furthermore, the water uptake evolution of each composite was monitored during the whole aging exposition. The experimental campaign evidenced that the beneficial effect of the sodium bicarbonate treatment on adhesion between flax fibers and epoxy matrix allows treated laminates to better retain their flexural properties during the salt-fog-exposition, in comparison to untreated laminates. Conversely, the proposed treatment leads to a slight worsening effect on the durability of jute fiber reinforced composites.

1. Introduction Marine industry has been paid a growing attention towards composite materials since the middle of the last century. Despite conventional glass fiber reinforced composites dominated the recreational boating industry for over 40 years, new materials such as natural fibers (i.e. lignocellulosic) begin to be used recently for semi-structural polymeric components useful for marine applications [1,2], to reduce the environmental impact and the end-of-life (EOL) cost disposal [3]. A wide range of natural fibers are nowadays employed as reinforcement of polymer based composites [4–6]. Among them, flax and jute fibers are very attractive mainly due to their specific mechanical properties and insulating characteristics. In particular, flax fiber was one of the first to be extracted, spun and woven into textiles [7] whereas jute can be considered the second most important fiber in terms of world production levels of natural fibers, next to cotton [8,9]. In addition to their inherent biodegradability, renewability and recyclability, the main advantages concerning natural fibers consist in their low density, low cost, good processability and large availability [3,4]. On the other hand, these fibers are also characterized by a large dispersion of their mechanical properties, which are strongly harvestdependent, influenced by climate, location, soil characteristics, weather circumstances as well as by fiber processing [10]. Moreover, they show ∗

lower mechanical properties than their synthetic counterparts such as glass, carbon and Kevlar fibers. Further drawbacks of natural fibers consist in their high moisture absorption and weak compatibility with several polymeric matrices [6,7], due to their hydrophilic nature. These features result in low ability to transfer stress from the matrix, dimensional changes of fibers that may lead to micro-cracking phenomena, thus lowering the mechanical properties of the resulting composites [11]. Furthermore, composite materials with weak fiber-matrix adhesion greatly suffer mechanical degradation when they are exposed, during their service-life, to humid environments such as marine one [12,13]. This behavior can be explained since the water diffusion within composite structures is favored through preferential pathways such as micro-cracks at the fiber/matrix interface. Consequently, composites experience great water uptake [14] and swelling phenomena due to the discrepancy in the water absorption of fibers and matrix [15]. Furthermore, fiber swelling can also damage the interface and the surrounding matrix, leading to premature delamination phenomena [16]. This represents a key point that limits the application fields of composite materials to specific operating conditions for which durability is not a limiting factor. A widely used approach aimed to increase the compatibility between natural fibers and polymeric matrices and, as a consequence, improve the mechanical response of the resulting composites, consists

Corresponding author. E-mail address: [email protected] (V. Fiore).

https://doi.org/10.1016/j.polymertesting.2019.106100 Received 11 July 2019; Received in revised form 27 August 2019; Accepted 3 September 2019 Available online 05 September 2019 0142-9418/ © 2019 Elsevier Ltd. All rights reserved.

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in fibers pretreatment. Several authors showed the beneficial effects of chemical [17–20] and physical methods [21–23] on the mechanical performances of natural fiber reinforced composites and their durability under humid environments [24–27]. In this context, an eco-friendly and cost-effective treatment based on the soaking of natural fibers in sodium bicarbonate solution has been carried out in the last years [28–34]. In particular, since a sodium bicarbonate solution is mildly alkaline due to the formation of carbonic acid and hydroxide ion, its interaction with the fiber surface can be considered similar to that which happens during a traditional mercerization treatment [28]. Starting from the experimental results obtained in an our previous paper [33], aim of the present work is to evaluate how the proposed treatment can improve the durability in marine environment of natural fiber reinforced composites. In more detail, since sodium bicarbonate treatment leads to the reduction of hemicellulose and lignin contents of natural fibers [28,29,34], two laminates reinforced with fibers having different amount of lignin such as flax (i.e., up to 4%) and jute (i.e., up to 25%) [6,7,35,36] were exposed to salt-fog environment up to 60 days (i.e., 2 months). The aging resistance against marine environment of the above laminates was assessed through tensile, flexural quasistatic tests and Charpy impact tests performed on both unaged and aged specimens (for 1 month and 2 months, respectively). Furthermore, the percentage of moisture uptake was evaluated for each laminate during the whole aging exposition.

un) show fiber volume fraction equal to about 30% while the voids content ranges from 3.17% up to 4.91%. 2.1. Salt-fog aging The durability of flax and jute fiber reinforced epoxy composites was evaluated by exposing all the manufactured laminates to salt-fog spray condition for a whole period of 2 months, according to ASTM B 117 standard. To this scope, a Weiss climatic chamber model SC/KWT 450 was used. In particular, the salt-fog had a chemical composition of 5% NaCl solution (i.e., pH between 6.5 and 7.2) and the temperature inside the climatic chamber was set to 35 °C. Single panels were taken out from the climatic chamber after 30 days and 60 days, respectively. From each panel, specimens were cut through a diamond saw in order to perform the mechanical characterization by means of tensile, flexural quasi-static tests and impact tests. The mechanical properties of the aged specimens were compared with those shown by the reference laminates (i.e., unaged ones) [33]. 2.2. Water uptake Before the exposition to the salt-fog environment, five specimens (100 mm × 100 mm) per laminate were dried in an oven at 50 °C and then were cooled to room temperature in a desiccator. Then, the above specimens were periodically removed from the climatic chamber, cleaned with a clean dry cloth and weighed by using an analytical balance, model AX 224 (Sartorius Italy). The percentage weight gain W was measured at different time intervals according to the following equation:

2. Material and methods The durability of the same composite laminates studied in our previous paper [33] was analyzed here. In particular, five laminae of flax or jute twill weave woven fabrics were used in the stacking sequence of the laminates. Flax fabrics, having areal weight 320 g/m2, were supplied by Lineo (France) while jute fabrics, having areal weight 400 g/m2, were supplied by Composites Evolution (UK). An epoxy resin SX8 EVO, supplied by Mates Italiana s.r.l. (Italy), mixed with its own amine-based M-type (medium reactivity) hardener (100:30 mix ratio by weight) was used as matrix. All the composite panels were manufactured through vacuum assisted resin infusion method, cured at 25 °C for 24 h and post-cured at 50 °C for 8 h. A two-stage vacuum pump was used to create maximum vacuum equal to 0.1 atm (absolute). In order to evaluate the effect of fiber treatment on composites durability, two laminates for each fiber (i.e. flax and jute) were produced by using untreated and treated fabrics. In more detail, flax and jute fabrics were soaked in a sodium bicarbonate solution (10 wt%) for 5 days at room temperature. Then, fabrics were washed with distilled water and dried at 40 °C for 24 h and at 103 °C for further 24 h, to remove the residual moisture content. Table 1 reports the reference code, the stacking sequence and the aging exposure time for each investigated batch. As stated in Ref. [33], all unaged laminates (i.e., Flax-AR-un; Flax-T-un; Jute-AR-un; Jute-T-

W (%) =

Stacking sequencea

fabrics

Aging time (days)

Flax-AR-un Flax-T-un Flax-AR-1m Flax-T-1m Flax-AR-2m Flax-T-2m Jute-AR-un Jute-T-un Jute-AR-1m Jute-T-1m Jute-AR-2m Jute-T-2m

[F]5 [F]5 [F]5 [F]5 [F]5 [F]5 [J]5 [J]5 [J]5 [J]5 [J]5 [J]5

as received treated as received treated as received treated as received treated as received treated as received treated

0 0 30 30 60 60 0 0 30 30 60 60

a

W0 W0

100

(1)

Where W0 and Wt are the initial weight and the weight at aging time t, respectively. 2.3. Quasi-static tensile tests Quasi-static tensile tests were carried out on five prismatic specimens (15 mm × 250 mm) for each condition, according to ASTM D 3039 standard. To this scope, an electromechanical Universal Testing Machine (U.T.M) model ETM-C by WANCE (China), equipped with a load cell of 50 kN, was used. The strain was evaluated through an YYU10/50 extensometer with gauge length of 50 mm. Tensile tests were performed in displacement control mode at a crosshead speed of 0.5 mm/min. 2.4. Quasi-static flexural tests Three point bending tests were performed according to ASTM D 790 standard, by using an electromechanical U.T.M. model Z005 by Zwick/ Roell (Germany), equipped with 5 kN load-cell. Five prismatic specimens (15 mm × 80 mm) were tested for each condition, setting the span length and the cross-head equal to 64 mm and 2 mm/min, respectively.

Table 1 List of manufactured composite laminates. Code

Wt

2.5. Charpy impact tests Impact tests were carried out according to EN ISO 179 standard, using a Charpy pendulum model 9050 from CEAST(Italy), equipped with a pendulum of potential energy equal to 5 J. Five un-notched prismatic specimens (80 mm × 10 mm) were tested for each condition at room temperature. 2.6. Morphological analysis Morphological analysis was performed on the tensile fractured

F = twill weave flax fabric; J = twill weave woven jute fabric. 2

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untreated jute fibers (i.e., Jute-AR), thus promoting higher moisture content at saturation. A further significant aspect that plays a relevant role in water sorption phenomena is related to the chemical composition of natural fibers since it affects the surface interaction with water and moisture environments. In particular, the main constituents of flax fibers are cellulose (i.e., ~71 wt%) hemicellulose (range from 18.6 to 20.6 wt%) and lignin (up to 4.0 wt%) [35]. Instead, the cellulose content in jute fibers ranges from 61 wt% to 71.5 wt%, hemicellulos from 13.6 to 20.4 wt% and lignin from 12 to 13 wt% [37]. Since cellulose and hemicellulose have a significant hydrophilic behavior (i.e., these compounds are characterized by numerous hydroxyl and carboxyl groups, which favor water molecules interaction via hydrogen bondings [7]), natural fibers with higher amount of these polysaccharides (such as flax fibers) are characterized by larger water absorption behavior [38]. After the fiber treatment with sodium bicarbonate, an inversion on moisture sensitivity can be highlighted. In more detail, Flax-T batch shows lower water uptake at saturation (i.e. ~10.5% after 840 h) than Jute-T laminates (i.e., ~13.5% after 840 h). Despite higher cellulose and hemicelluloses contents within flax fibers, Flax-T composites experience lower moisture content than their counterparts (i.e. Jute-T) since the eco-friendly treatment allowed to improve the adhesion between flax fibers and epoxy matrix while a slight worsening effect was observed for jute fabrics [33]: i.e. the weaker fiber-matrix adhesion for jute treated laminates compared to flax promotes its higher moisture content at saturation.

Fig. 1. Weight gain versus square root of aging time curves.

surfaces by using an environmental scanning electron microscopy (ESEM) model Quanta 450 by FEI (OR, USA) with an accelerating voltage of 5 kV. Before analysis, each specimen was sputter-coated with a thin layer of gold to avoid electrostatic charging under the electron beam. 3. Results and discussion 3.1. Water uptake

3.2. Quasi-static tests

The percentage weight gain for each laminate is shown as a function of square root of aging time in Fig. 1. Initially, the water absorption quickly increased with a quite linear trend for all the investigated batches. Afterwards, the water uptake process slows and approaches saturation over prolonged time, (i.e., a plateau is reached after about 700–850 h). By analyzing the trend for as received batches (green filled markers in Fig. 1), it is worth noting that Flax-AR laminates show higher maximum water absorption (i.e., ~14% after 700 h) than Jute-AR laminates (i.e., ~11.5% after 840 h). These results indicate the greater sensitivity to moisture interaction and diffusion of flax laminates in comparison to jute reinforced ones. In particular, during the first uptake stages, a bilinear trend can be detected in the water absorption curve. This behavior can be associated with the different mechanisms involved in the process of water absorption in the composite laminates. Each mechanism is characterized by different sorption kinetic phenomena. The fastest one plays a key role in the first segment, while the kinetically slower processes have a significant contribution, at intermediate times, before reaching saturation. An important contribute, influencing the water absorption behavior of natural fiber reinforced composites exposed to humid environments, can be related to their fiber-matrix adhesion. In that respect, it was shown [33] that Flax-AR laminates are characterized by weaker fibermatrix interfacial adhesion in comparison to laminates reinforced with

Table 2 reports the quasi-static properties of jute laminates for each investigated aging condition. It was clearly evidenced in an our previous paper [33] that a slight worsening of adhesion between jute fibers and epoxy resin is achieved after sodium bicarbonate treatment, thus leading to small decrements of tensile and flexural strength of Jute-T-un laminates in comparison to those of Jute-AR-un ones. At the same time, tensile and flexural moduli remain almost constant whereas an overall decrease of the strain at break occurs both in tensile and in flexural configuration, after the fiber treatment. In order to highlight the effect of salt-fog exposition on the quasistatic properties of jute laminates, the percentage reductions of tensile and flexural properties of the aged laminates in comparison to unaged reference are shown in Fig. 2. In particular, Fig. 2a shows that, after 2 months of salt-fog exposition, noticeable decrements of the flexural strength are experienced by jute laminates due to the aging exposition: i.e., −17.1% and −39.1% for Jute-AR and Jute-T laminates, respectively. This detrimental effect takes place also at low aging time. Indeed, after 1 month of salt-fog exposition Jute-T and Jute-AR laminates show −32% and −15.8% lower tensile strength than unaged specimens, respectively. Similarly, Jute-T laminates evidence greater decrement of the flexural modulus (i.e., −64.9%) than that experienced by Jute-AR laminates (i.e.,

Table 2 Quasi-static properties of jute laminates for each aging condition. Aging days

Jute-AR

Jute-T

Tensile Strength [MPa] 0 30 60

59.3 ± 1.9 57.8 ± 3.2 56.7 ± 4.0

87.6 ± 3.7 73.8 ± 1.7 72.6 ± 1.7

Flax-T

Tensile Modulus [GPa] 56.4 ± 3.1 54.2 ± 2.1 53.3 ± 1.5

Flexural Strength [MPa] 0 30 60

Jute-AR

5.83 ± 0.18 5.15 ± 0.20 4.59 ± 0.74

4.36 ± 0.19 3.75 ± 0.12 3.39 ± 0.38

3

Flax-T

Strain at break [%] 5.75 ± 0.37 4.85 ± 0.25 3.98 ± 0.53

Flexural Modulus [GPa] 80.8 ± 3.7 54.9 ± 4.6 49.2 ± 2.4

Jute-AR

1.59 ± 0.07 1.75 ± 0.08 1.85 ± 0.10

1.42 ± 0.20 1.81 ± 0.15 2.21 ± 0.33

Strain at break [%] 4.42 ± 0.12 2.08 ± 0.14 1.55 ± 0.22

2.98 ± 0.14 3.51 ± 0.21 3.63 ± 0.26

2.41 ± 0.12 4.76 ± 0.35 5.85 ± 1.07

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Fig. 2. Percentage reductions of the quasi-static properties of jute laminates for each aging condition.

−22.2%) at the end of the salt-fog exposition (Fig. 2c). As a consequence, the strain at break of Jute-T-2m samples (i.e., 5.85%) is 143% higher than that of Jute-T-un ones (i.e., 2.41%) whereas Jute-AR-2m samples show strain at break (i.e., 3.63%) only 22% higher than that of Jute-AR-un ones (i.e., 2.98%) The evolution of the flexural properties of Jute laminates during the salt-fog exposition can be explained by considering the effect of sodium bicarbonate treatment on fiber-matrix adhesion. In particular, since the as received specimens (i.e. Jute-AR) showed better interfacial adhesion than Jute-T ones [33], these last evidenced higher water absorption (see Fig. 1) during their exposition in the salt-fog environment: i.e., the moisture penetrates through voids and cracks within the epoxy matrix and mainly through the weak interface surfaces. This leads, in turn, to the weakening of both matrix and natural fibers and subsequent local debonding and swelling phenomena [42]. In addition, the removal of surface impurities (such as wax and fatty compounds) coupled with the reduction of lignin and hemicellulose components from the fiber bulk due to the soaking of jute fibers in the NaHCO3 solution [29] favor the activation and propagation of damage phenomena, thus leading to a more effective damage sensitivity in severe environmental conditions [39]. For these reasons, Jute-T laminates show greater decrements of flexural strength and modulus as well as lower decrement of the strain at break in comparison to Jute-AR laminates, during the aging exposition. Furthermore, it is worth noting that jute laminates show, at the end of the salt-fog exposition, a slight reduction of tensile strength and modulus, in comparison to the corresponding variations of the flexural properties. In particular, Fig. 2b shows that, after 2 months of salt-fog exposition, the reduction of tensile strength of jute laminates is less than 10% for both composites (i.e., −4.5% and −5.4% for Jute-AR and Jute-T laminates, respectively). On the other hand, the reductions of the tensile modulus was found equal to −21.3% and −30.8%, for Jute-AR and Jute-T laminates, respectively (Fig. 2d). These results can be possibly explained by considering the different effect of the fiber-matrix adhesion on tensile and flexural maximum stress performances of the resulting laminates. In more detail, it is widely known that the flexural strength of composite laminates is more influenced than its tensile strength by the quality of the fiber-matrix

interface [40]. The structural behavior of composite laminates is related to the different stress states applied. Under three point flexural loads, lower and upper surfaces of the sample suffer bending stress (i.e., tensile and compressive stresses, respectively) whereas shear stress takes place in the axisymmetric plane. Therefore, two competing failure modes (i.e., flexural and shear failure) contribute to the final fracture of the laminate. A linear elastic behavior followed by catastrophic fracture can be usually related to flexural failure mode. Instead, if large deformation and non-linear stress-strain trend is observed, a mixed failure mode takes place. In this case, the shear deformation contribution plays a relevant role in the final failure fracture of the composite laminate [41,42]. Therefore, flexural tests, despite tensile ones, imply a combined compressive/tensile and interfacial shear stresses that favor local mechanical instability mainly for composite laminates with weak interfacial adhesion: i.e., the stress state induced by a flexural load implies interlaminar shear stresses for which interfacial adhesion plays a fundamental role. Due to these reasons, both jute laminates (i.e., regardless the fiber treatment) experience low decrements of the tensile strength after 2 months of salt-fog exposition whereas Jute-T laminates show a greater decrement (i.e., about twice) of the flexural strength than that of JuteAR ones, at the end of the aging campaign. In comparison to jute reinforced laminates, flax laminates show different trends of their quasi-static properties over aging time, as reported in Table 3. In particular, thanks to the enhancement of the fiber-matrix adhesion due to sodium bicarbonate treatment, it was shown in our previous paper [33] that unaged laminates reinforced with treated fibers (i.e., Flax-T-un) evidence higher average flexural strength than those reinforced with treated fibers (i.e., Flax-AR-un). As a consequence of the enhanced fiber-matrix compatibility, the flexural strength reduction showed by Flax-T laminates during the salt-fog exposition (i.e., −27.1%) is lower than that of Flax-AR laminates (i.e., −50.3%), as evidenced in Fig. 3a. Similar considerations can be argued by observing the flexural modulus values: (i) with regard to unaged specimens, Flax-T laminates evidence higher average value than Flax-AR ones; (ii) laminates 4

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Table 3 Quasi-static properties of flax laminates for each aging condition. Aging days

Flax-AR

Flax-T

Tensile Strength [MPa] 0 30 60

70.7 ± 2.4 68.5 ± 2.0 66.7 ± 2.1

73.7 ± 6.0 47.2 ± 4.3 36.7 ± 1.7

Flax-T

Tensile Modulus [GPa] 76.0 ± 2.7 69.8 ± 3.0 65.1 ± 2.5

Flexural Strength [MPa] 0 30 60

Flax-AR

4.94 ± 0.32 3.56 ± 0.28 2.75 ± 0.25

3.75 ± 0.47 1.34 ± 0.13 1.36 ± 0.09

reinforced with treated flax fibers (i.e. Flax-T) better retain their flexural stiffness (i.e., −37.6%) than Flax-AR laminates (i.e., −63.8%) during the aging exposition (Fig. 3c). As already stated, these experimental results can be mainly explained by the beneficial effect on the fiber–matrix adhesion due the soaking of flax fibers in the sodium bicarbonate solution. Furthermore, since a weak or strong fiber-matrix adhesion influences the flexural response of fiber reinforced composites to a greater extent in comparison to their tensile behavior, no noticeable differences are observed between the quasi-static tensile properties of laminates reinforced with untreated and treated flax fibers. More specifically, the tensile strength and modulus of Flax-AR-2m specimens are −5.7% and −44.4% lower than the corresponding values of the unaged specimens (i.e., Flax-AR-un), respectively. On the other hand, Flax-T laminates experience decrements of their tensile strength and modulus equal to −14.3% and −51.1% at the end of the aging campaign, respectively (Fig. 3b,d). Overall, the enhancement of the interfacial adhesion between flax fibers and epoxy matrix, due to the fiber surface treatment, can be traced by comparing the mechanical properties evolution at increasing aging time under tensile or bending tests, respectively. In more detail, under tensile test, the sodium bicarbonate treatment does not imply a significant change in the durability of the composite laminates.

Flax-T

Strain at break [%] 7.40 ± 0.23 4.15 ± 0.30 3.62 ± 0.25

Flexural Modulus [GPa] 103.5 ± 7.3 80.6 ± 1.7 75.5 ± 2.0

Flax-AR

2.63 ± 0.11 3.83 ± 0.13 5.37 ± 0.35

1.54 ± 0.10 3.05 ± 0.21 3.67 ± 0.28

Strain at break [%] 5.48 ± 0.23 3.47 ± 0.35 3.42 ± 0.44

5.23 ± 0.19 8.37 ± 0.64 8.90 ± 0.73

2.98 ± 0.25 6.63 ± 0.19 6.54 ± 0.41

Conversely, for flexural tests, for which the interlaminar shear stresses contribute may be significant [43], there is an increase in mechanical stability under severe environmental conditions thanks to the better interfacial adhesion which limits the water diffusion and the subsequent delamination and swelling phenomena. This behavior can be justified considering the different interaction of the compared natural fibers in NaHCO3 solution. In particular, flax fibers are constituted by lower content of lignin (i.e., up to 4%) than jute (i.e., up to 25–27%) [35,37]. In addition to the impurities removal [32], the soaking in sodium bicarbonate solution leads to the surface dissolution of hemicellulose and lignin thus favoring, in turn, the modification of the ligand/fibrils ratio in the natural fiber microstructure [29]. Therefore, sodium bicarbonate treatment could imply a softening effect on fibers with high lignin content (i.e. jute fibers). This results in a relevant sensitivity to degradation in severe environmental condition of the Jute-T laminates compared to Jute-AR laminates. Conversely, for flax fibers having low lignin content and compact structure, the sodium bicarbonate solution is not able to create large surface damaging and the structure integrity of the vegetable fiber is preserved [44]. This leads to a reliable and suitable mechanical stability of the Flax-T batch during aging in salt-fog environment.

Fig. 3. Percentage reductions of the quasi-static properties of flax laminates for each aging condition. 5

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3.3. Impact tests

at fiber-matrix interface where the weak adhesion triggers preferential pathways for water diffusion, leading to a progressive softening of the resin bulk [52,53]. Thanks to the enhanced fiber-matrix compatibility due to NaHCO3 treatment, Flax-T laminates evidence an unexpected increase of their peak load (i.e., from 180.6 N to 227.9 N) in addition to a feeble increase of the deflection at break (i.e., from 4.2 mm to 5.1 mm) by increasing the exposition time. This results in a good stability of the impact performances shown by epoxy laminates reinforced with treated flax fibers against salt-fog environment (i.e. the impact energy of the 2-months aged laminates is about 60% higher than that of unaged specimens). Conversely, in the Flax-AR laminates a slight reduction in the peak load (i.e., from 130.5 N to 103.3 N) is coupled with a relevant increase in the deflection at break (i.e., from 7.7 mm to 27.0 mm), as a consequence of a limited interfacial adhesion, with subsequent relevant increase of impact energy (i.e. +164.7%) after 60 days of salt-fog aging. A deeper understanding of the impact behavior evolution of flax laminates during the salt-fog exposition can be extrapolated by observing the typical load-deflection curves shown in Fig. 4. The area under the load-deflection curve, that is proportional to the impact energy, can be used as visual index of the impact performances of laminates with increasing aging time. As already stated, a large increase in impact energy for Flax-AR laminates occurs at increasing aging time, mainly due to the significant increase of the deflection at break (Fig. 4a). Conversely, the area under the load/deflection curves remains almost constant for the Flax-T batch, regardless the salt-fogexposition time. A mainly brittle behavior, with a quite linear trend until the failure, can be highlighted. Furthermore, the deflection at break is about one third of that observed for the Flax-AR laminates. These results are in good agreement with the quasi-static tests for which a stiffening effect in the composite laminate induced by the treatment with sodium bicarbonate of the flax fibers takes place. As previously shown [33], Jute-AR-un and Jute-T-un laminates show similar values of their impact energy (i.e., 13.5 kJ/m2 vs 14.7 kJ/ m2), due to the feeble effect of sodium bicarbonate treatment on the adhesion between jute fibers and the epoxy matrix. Regarding to the effect of the salt-fog exposition, it is worth noting that a noticeable increase in the impact properties of laminates reinforced with treated jute fibers happens at increasing exposition time to salt-fog. Indeed, Jute-T laminates show an impact energy of 58.7 kJ/ m2 after 60 days of salt-fog aging, approximately 300% higher than that of unaged laminates (i.e., 14.7 kJ/m2). Furthermore, a reduction in the peak load (i.e., from 223.6 N to 110.5 N) and a significant increase in the deflection at break (i.e., from 3.7 mm to 23.9 mm), occur after 60 days of salt-fog aging. These results further confirm the slight detrimental effect of the proposed treatment on the adhesion between jute fibers and the epoxy resin. As already discussed, treated jute laminates show higher water absorption at saturation due to their weak fiber/matrix adhesion (Fig. 1) [33]. This low compatibility implies that the water diffusion is kinetically favored at the matrix-fiber interface with a consequent softening

It is well known that when fiber-reinforced composites undergo an impact load, the energy is dissipated by the combination of several damaging phenomena such as fiber pull-outs, fiber fractures and/or matrix deformations and fractures [45]. Fiber fracture, failure mechanism typical for composites having strong interfacial adhesion, dissipates less energy compared to fiber pull-outs that conversely occur more frequently when the fiber-matrix bond is weak. Hence, a strong fiber-matrix adhesion due to, for instance, fiber treatments reduces the impact energy capability of composites [46,47]. As discussed in our previous paper [33], the impact energy of FlaxAR-un laminates is higher than that of jute ones, due to the greater toughness of flax dry fabrics in comparison to their counterparts. Furthermore, after the sodium bicarbonate treatment jute unaged laminates don't evidence any remarkable variation of their impact energy. This means that no relevant improvement of the adhesion between jute fibers and epoxy matrix was achieved through the sodium bicarbonate treatment. Conversely, a relevant decrease of the impact energy was shown by flax laminates after the NaHCO3 treatment, thus confirming the positive effect of the above treatment on flax-epoxy adhesion [48]. With regard to the aging effect, two competing phenomena usually contribute to the impact energy variation of natural fiber reinforced composites exposed to humid environmental conditions such as salt-fog exposition [49,50]: (i) a reduction of the maximum load capability, due to chemical or physical degradation phenomena within the laminate induced by salt-fog; (ii) an increase of the maximum deformation capability, mainly due to the water absorption within both the epoxy matrix and natural fibers (i.e., flax or jute). The former acts as detrimental contribute on impact energy capability. Vice versa, the latter plays a key role on the impact performance improvement of composite laminates. Depending on which of the above mentioned mechanisms is predominant, it is possible to have an increase or decrease in the impact performances at increasing aging time [50]. By observing the evolution of the impact properties shown in Table 4, it is worth noting that all the investigated laminates improve their impact energy at increasing the exposition time to salt-fog. With regard to flax reinforced laminates, it was found that Flax-T laminates improve their impact energy less than Flax-AR ones (i.e. +61.2% vs + 164.7%, respectively) at the end of aging exposition (i.e., after 60 days). This behavior can be explained taking into account both the hydrophilic nature of untreated flax fibers (i.e., due to high amount of cellulose and hemicellulose) and their weak adhesion with the epoxy matrix, thus promoting higher moisture content at saturation of FlaxAR laminates than Flax-T ones (as shown in Fig. 1). Indeed, under humid environments such as salt-fog, untreated flax fibers are likely to absorb great amount of water mainly due to the weak fiber-matrix adhesion. In such a way, the reinforcing fibers become more ductile which consequent enhancement of toughness performances of the composite [51]. Furthermore, a softening of the epoxy resin could occur Table 4 Impact properties of laminates for each aging condition. Aging days

Flax-AR

Flax-T

Flax-AR

Impact Energy [KJ/m2] 0 30 60

0 30 60

Flax-T Peak Load [N]

Flax-AR

Flax-T

Deflection at break [mm]

21.5 ± 2.9 45.1 ± 7.5 57.0 ± 5.5

12.8 ± 2.1 19.7 ± 1.9 20.7 ± 1.2

130.5 ± 5 125.5 ± 9.5 103.3 ± 10.7

180.6 ± 13.1 225.3 ± 8.15 227.9 ± 4.5

7.7 ± 0.5 14.2 ± 2.1 27.0 ± 2.5

4.2 ± 0.3 4.8 ± 0.3 5.1 ± 0.3

Jute-AR

Jute-T

Jute-AR

Jute-T

Jute-AR

Jute-T

13.5 ± 0.9 16.1 ± 1.6 16.8 ± 1.7

14.7 ± 3.1 51.2 ± 8.1 58.7 ± 6.4

284.8 ± 25.7 226.2 ± 23.8 193.5 ± 10.3

223.6 ± 64 123.3 ± 7.8 110.5 ± 3.2

2.9 ± 0.3 4.2 ± 0.3 5.0 ± 0.4

3.7 ± 0.3 17.7 ± 2.8 23.9 ± 0.9

6

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Fig. 4. Typical load-deflection impact curves at varying aging time exposition of (a) Flax-AR and (b) Flax-T laminates.

of jute treated fibers. Moreover, the interfacial matrix layer shows a progressive toughening effect (i.e., increase of the deflection at break) thus leading to leads to the impact energy increase. On the other hand, the impact energy of Jute-AR laminates remains almost constant after the salt-fog exposition: Jute-AR-2m laminates show an impact energy of 16.8 kJ/m2, 24.4% higher than that of unaged laminates (i.e., 13.5 kJ/m2). This result is due to both a slight reduction in the peak load (i.e., from 284.8 N to 193.5 N) and a small increase in the deflection at break (i.e., from 2.9 mm to 5.0 mm), experienced by laminates reinforced with as received jute fabrics occur after 60 days of salt-fog aging. Further details concerning the impact behavior evolution of jute laminates at increasing exposition time in the salt-fog environment can be acquired by analyzing the typical load-deflection curves of unaged and aged specimens, shown in Fig. 5. As evidenced in Fig. 5a, laminates reinforced with untreated jute fibers show moderate reductions in strength and stiffness at increasing aging time. Vice versa, the surface treatment induced a very relevant dynamic mechanical instability in jute composite laminate (Fig. 5b). Jute-T-1m specimen, compared to Jute-T-un one, shows appreciable different mechanical response to the dynamic loading. The material already after 1 month in the salt-fog climatic chamber shows an evident loss of stiffness, due to softening and plasticization phenomena of fiber and matrix [52] Indeed, different combined chemo-physical phenomena (i.e., plasticizing, swelling, and hydrolysis) could occur by hydrothermal aging [54,55]. In natural fiber reinforced composites, these phenomena occur toward three main preferential paths for moisture flow: i) water diffusion through fiber lumen [56]; ii) capillary diffusion at the fiber/matrix interface and iii) transport through the polymeric matrix [57]. This implies a significant increase in impact energy for Jute-T laminates with increasing aging time. This occurs even if the maximum load is reduced, due to a large increase in the deformation at break, which leads to an increase in the area under the load/displacement curve (Fig. 5b). These considerations are in agreement with the results of the quasi-static tests, although the mechanical

impulsive phenomenon amplifies the influence of the hydrothermal degradation in the salt-fog chamber in the mechanical stability of the jute-based composite laminate. The morphology of the fractured surfaces of untreated and treated jute and flax composite laminates after aging tests is shown in Fig. 6. Flax-AR sample (Fig. 6a) is characterized by wide dispersed fractures, due to debonding phenomena, triggered and propagated at the fiber/ matrix interface. Furthermore, exhausted bundles can be highlighted. Conversely, Flax-T sample (Fig. 6b) exhibits a better interfacial adhesion evidencing a fracture surface with limited debonded areas. More fiber bundles are embedded and not exhausted confirming the good interaction between flax fibers and epoxy matrix after the proposed treatment. These different morphologies allow to clarify the previously discussed mechanical results: i.e., due to the sodium bicarbonate treatment, flax fibers enhances their chemo-mechanical compatibility with the epoxy matrix, thus leading to a consequent strengthening and stiffening of the resulting composites [31]. As a consequence, Flax-T laminates show an improved durability under critical environmental conditions in comparison to that of Flax-AR ones. As shown in Fig. 6c, the fracture surface of Jute-AR is quite compact with some surface grooves due to debonding of fiber bundles. This morphology suggests, also after aging tests, a suitable adhesion of as received jute fabrics with the epoxy resin. Nevertheless, few local debonded areas and some matrix cracks can be identified in the fracture surface. The presence of matrix cracks can be considered as a confirmation of the good adhesion between composite constituents. The stress transfer at the fiber/matrix interface is effective leading to fracture evolution by crack propagation in the matrix bulk rather than due to debonding phenomena. On the other hand, a detrimental effect of the surface treatment on the jute fiber-matrix adhesion can be argued by analyzing the fracture surface of the aged Jute-T sample (Fig. 6d). Indeed, some local deboned areas and pull-out phenomena can be identified. This morphology results as a consequence of the lower adhesion at the fiber/matrix interface caused by surface treatment. The weak fiber-matrix adhesion, stimulated by salt-fog exposition, leads to a

Fig. 5. Typical load-deflection impact curves at varying aging time exposition of (a) Jute-AR and (b) Jute-T laminates. 7

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Fig. 6. SEM micrographs of fracture surfaces of composite laminates after salt-fog exposition: (a) Flax-AR, (b) Flax-T, (c) Jute-AR and (d) Jute-T.

In conclusion, this paper evidenced that the sodium bicarbonate treatment allows flax fiber reinforced composites to improve their aging resistance against salt-fog environmental conditions. Conversely, the proposed treatment leads to a slight worsening effect on jute based composites durability. These statements were also confirmed by morphological SEM analysis.

premature failure of the Jute-T laminate at lower stress levels compared to untreated laminate, as observed by mechanical tests. 4. Conclusions In the present paper, flax and jute fibers were treated by means of an innovative and eco-friendly chemical treatment based on sodium bicarbonate solution (10 wt%) in order to improve the aging resistance against salt-fog environment of natural fiber reinforced epoxy composite laminates. In particular, the evolution of their mechanical properties and water uptake was monitored during the aging in a salt-fog environment. The main results, acquired by the experimental campaign, can be summarized in the following:

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymertesting.2019.106100. References [1] C. Fragassa, Marine applications of natural fibre-reinforced composites: a manufacturing case study, Adv. Appl. Ind. Biomater, Springer International Publishing, Cham, 2017, pp. 21–47, , https://doi.org/10.1007/978-3-319-62767-0_2. [2] M.P. Ansell, Natural fibre composites in a marine environment, Nat. Fibre Compos. Mater. Process. Appl, Woodhead Publishing, 2013, pp. 365–374, , https://doi.org/ 10.1533/9780857099228.3.365. [3] J. Summerscales, M.M. Singh, K. Wittamore, Disposal of composite boats and other marine composites, Mar. Appl. Adv. Fibre-Reinforced Compos, Woodhead Publishing, 2015, pp. 185–213, , https://doi.org/10.1016/B978-1-78242-250-1. 00008-9. [4] J. Summerscales, N.P.J. Dissanayake, A.S. Virk, W. Hall, A review of bast fibres and their composites. Part 1 – fibres as reinforcements, Compos. Part A Appl. Sci. Manuf. 41 (2010) 1329–1335, https://doi.org/10.1016/j.compositesa.2010.06. 001. [5] H.M. Akil, M.F. Omar, A.A.M. Mazuki, S. Safiee, Z.A.M. Ishak, A. Abu Bakar, Kenaf fiber reinforced composites: a review, Mater. Des. 32 (2011) 4107–4121, https:// doi.org/10.1016/j.matdes.2011.04.008. [6] F. Sarasini, V. Fiore, A systematic literature review on less common natural fibres and their biocomposites, J. Clean. Prod. 195 (2018) 240–267, https://doi.org/10. 1016/j.jclepro.2018.05.197. [7] L. Yan, N. Chouw, K. Jayaraman, Flax fibre and its composites – a review, Compos. Part B Eng. 56 (2014) 296–317, https://doi.org/10.1016/j.compositesb.2013.08. 014. [8] A.K. Mohanty, M. Misra, Studies on jute composites—a literature review, Polym. Plast. Technol. Eng. 34 (1995) 729–792, https://doi.org/10.1080/ 03602559508009599. [9] E. Gogna, R. Kumar, Anurag, A.K. Sahoo, A. Panda, A comprehensive review on jute fiber reinforced composites, Lect. Notes Mech. Eng, Springer, Singapore, 2019, pp. 459–467, , https://doi.org/10.1007/978-981-13-6412-9_45. [10] G. Di Bella, V. Fiore, A. Valenza, Effect of areal weight and chemical treatment on the mechanical properties of bidirectional flax fabrics reinforced composites, Mater. Des. 31 (2010) 4098–4103. [11] T. Scalici, V. Fiore, A. Valenza, Effect of plasma treatment on the properties of Arundo Donax L. leaf fibres and its bio-based epoxy composites: a preliminary

− Laminates reinforced with treated jute fibers (i.e. Jute-T) show higher water uptake at saturation (i.e., 13.5%) than Jute-AR ones (i.e., 11.5%) whereas, for flax based composites, the proposed treatment allows to decrease the maximum water absorption (i.e., 10.5% and 13.5% for Flax-T and Flax-AR laminates, respectively); − Flax-T laminates better retain their flexural modulus and strength (i.e., −37.6% and −27.1%, respectively) than Flax-AR laminates (i.e., −63.8% and −50.3%, respectively) during the aging exposition; − Jute-T laminates experience a higher reductions of the flexural modulus (i.e., −64.9%) and strength (i.e., −39.1%) than Jute-AR laminates (i.e., −22.2% and −17.1%, respectively) after 2 months of salt-fog aging. − Jute-T laminates show a relevant increase of their impact energy after 60 days of salt-fog aging (300% higher than that of unaged laminates, mainly due to a significant increase in the deflection at break) whereas the impact energy of Jute-AR laminates remains almost constant during the aging exposition, due to the small reductions of both peak load and deflection at break; − Flax-T laminates improve their impact energy less than Flax-AR ones (i.e. +61.2% vs +164.7%, respectively) at the end of aging exposition. Flax-T laminates evidence an unexpected increase of their peak load in addition to a feeble increase of the deflection at break. Conversely, in the Flax-AR laminates a slight reduction in peak load is coupled with a relevant increase of deflection at break. 8

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