Dynamic Mechanical Behavior Analysis of Flax/Jute Fiber-Reinforced Composites Under Salt-Fog Spray Environment
polymers
Article Dynamic Mechanical Behavior Analysis of Flax/Jute Fiber-Reinforced Composites under Salt-Fog Spray Environment
Vincenzo Fiore 1,* , Carmelo Sanfilippo 1 and Luigi Calabrese 2 1 Department of Engineering, University of Palermo, Viale delle Scienze, Edificio 6, 90128 Palermo, Italy; carmelo.sanfi[email protected] 2 Department of Engineering, University of Messina, Contrada Di Dio (Sant’Agata), 98166 Messina, Italy; [email protected] * Correspondence: vincenzo.fi[email protected]; Tel.: +39-091-23863721
Received: 13 February 2020; Accepted: 20 March 2020; Published: 24 March 2020
Abstract: Over the last decades, natural fiber-reinforced polymer composites (NFRPs) gained great attention in several engineering fields thanks to the reduction of the environmental impact and the end-of-life cost disposal. Unfortunately, the use of NFRPs is limited, mainly due to their weak resistance against humid environments. Since limited literature is available about the evolution of the dynamic mechanical response of NFRPs under aggressive environments, this paper aims to investigate the damping properties of flax, jute and flax/jute epoxy composites exposed to salt-fog up to 60 days. Furthermore, sodium bicarbonate fiber treatment was performed to improve the composites’ durability. The effectiveness of treatment was evidenced for full flax-reinforced composites, whereas no beneficial effect was found for jute composites. Moreover, treated hybrid laminates having outer laminae reinforced with flax showed better damping behavior than their hybrid counterparts during the whole aging campaign.
Keywords: damping; green composites; salt-fog exposition; aging behavior; surface treatment
1. Introduction Among natural fibers, flax [1] and jute [2] are nowadays widely utilized instead of synthetic counterparts such as glass in fiber-reinforced polymers (FRPs). Thanks to their specific mechanical properties [3] and good insulating features [4], flax and jute fibers have received great attention as reinforcements of composite components useful in several engineering applications [5–9]. Natural fiber-reinforced composites (NFRPs) have received growing attention in the last decades, both from the academic world and from industries, thanks to their specific mechanical properties, reduced cost and advantages for health and recyclability. Nevertheless, this class of innovative materials shows also some drawbacks, such as their limited aging resistance in humid or wet environmental conditions, i.e., an extensive literature shows that NFRP materials experienced dramatic degradation of their quasi-static mechanical properties when exposed to these environments [10–18]. This can be partially ascribed to the softening and plasticization phenomena of natural fibers and thermoset matrix [19,20]. In particular, different combined chemo-physical phenomena (i.e., plasticizing, swelling, and hydrolysis) occur in organic resins due to hydrothermal aging [21–23]. Furthermore, it is widely known that natural fibers show a high tendency to moisture absorption [24,25] due to the hydrophilic nature of their polysaccharide constituents (i.e., mainly cellulose and hemicellulose). In more detail, hemicellulose and the amorphous fraction of cellulose absorb water (due to the high percentage of hydroxyl groups), thus leading to a decrease of the tensile stability of natural fibers that become more flexible as a result of the plasticization effect [20,26].
Polymers 2020, 12, 716; doi:10.3390/polym12030716 www.mdpi.com/journal/polymers Polymers 2020, 12, 716 2 of 18
In addition, the degradation phenomena of natural fibers are accelerated in seawater due to + the presence of salts such as NaCl [27]. This latter is dissolved in the form of Na cations and Cl− anions, which, being able to spread within the composite structure, favor local damages especially in its interstitial areas. This mechanism also stimulates an osmotic water diffusion at the fiber/matrix interface, which further speeds up the interfacial debonding phenomena [28,29]. Another issue that contributes to a great extent to the low aging resistance of NFRPs under humid environmental conditions is the poor compatibility between hydrophilic natural fibers and several hydrophobic polymeric matrices such as epoxy resins [30,31]. In this regard, some researchers showed that the water resistance of natural fiber-reinforced polymer composites can be improved and their quasi-static mechanical properties under humid environment can be better retained through fiber chemical treatments [32–35]. Despite the wide literature about the quasi-static properties, research concerning the evolution of the dynamic mechanical behavior of NFRPs under such critical environmental conditions [36–39] and mainly how fiber surface treatments can help these materials to retain their damping properties [40,41] is still limited to date. It is widely known that the knowledge of the damping properties such as storage modulus (E’), loss modulus (E”), and loss factor (tan δ) provides essential information about the interfacial bonding between reinforcing fibers and the polymer matrix of composite materials [42]. For this reason, dynamic mechanical thermal analysis (DMTA) represents an indispensable and effective tool for analyzing the behavior of NFRPs under aggressive environments. Moreover, this characterization can furnish the designer with important data about the effectiveness of fiber treatments. Within this context, the beneficial effect of surface treatments such as alkalization, silanization, acetylation and alkali-silanization on the dynamic mechanical behavior of flax fiber-reinforced polymers (FFRPs) under hydrothermal aging was shown by Wang and Petru [40]. The water resistance and damping properties of FFRPs were improved after all surface treatments, even if the acetylation treatment led the best damping performance among all treated composites. Analogously, Doan et al. [41] showed that the dynamic mechanical behavior of jute/polypropylene composites under humid environmental conditions (i.e., 20 ◦C; 95% RH) can be improved by adding to the polymeric matrix an appropriate modifier (i.e., maleic anhydride-grafted polypropylene). Despite their beneficial effects on the fiber–matrix adhesion and, consequently, on the static and dynamic mechanical properties of NFRP materials, it is worth noting that the chemical methods performed in the past are considered expensive and/or harmful to the environment because of the use of hazardous chemical reagents. To overcome this problem, an eco-friendly and cost-effective treatment consisting of the immersion of natural fibers in sodium bicarbonate solution was investigated in the last years by several researchers [32,43–49]. In particular, they showed that the interaction between the mildly alkaline solution and the fiber surface is considered similar to that of traditional alkalization treatment [43]. Even if the beneficial effect of sodium bicarbonate treatment on the mechanical properties of NFRPs was widely evidenced, no one has yet investigated how this new approach allows better retention of the dynamic mechanical stability of NFRPs under aggressive environments. Within this scope, the effectiveness of sodium bicarbonate treatment on the evolution of the dynamic mechanical properties of flax, jute and hybrid flax/jute fiber-reinforced composites under salt-fog environment was evaluated for the first time in the present paper. Epoxy composites reinforced with untreated (i.e., as received) and treated fibers were exposed to salt-fog environments and their damping properties were evaluated after 30 and 60 days of aging, respectively.
2. Materials and Methods In the present paper the evolution of the dynamic mechanical properties of flax, jute and hybrid flax/jute fiber-reinforced epoxy laminates [48] was analyzed by increasing their exposition time in a salt-fog environment. In particular, two kinds of balanced 2 2 twill weave woven fabrics were × Polymers 2020, 12, 716 3 of 18 used as reinforcement, i.e., flax (areal weight 320 g/m2) and jute fabrics (400 g/m2), supplied by Lineo (Valliquerville, France) and Composites Evolution (Chesterfield, UK), respectively. SX8 EVO A epoxy resin, supplied by Mates Italiana s.r.l. (Segrate-Milano, Italy), mixed with SX8 EVO B hardener (100:30 mix ratio by weight) was used as a matrix. A vacuum-assisted resin infusion method was employed as a manufacturing method of composite panels, cured at 25 ◦C for 24 h and post-cured at 50 ◦C for 8 h. The untreated fabrics were dried at 40 ◦C for 24 h and at 103 ◦C for further 24 h to remove the moisture content. To pre-treat the natural reinforcement, flax and jute fabrics were soaked in a 10 wt.% distilled water sodium bicarbonate (i.e., NaHCO3) solution at 25 ◦C for 5 days, washed with distilled water and dried at 40 ◦C for 24 h and at 103 ◦C for further 24 h. All the compared laminates (see Table1) showed fiber volume fraction equal to about 30%, while the voids content was lower than 5%, as reported in our previous paper [48]. In particular, the voids volume content (vV) of each composite was evaluated by comparing its experimental and theoretical densities:
ρt ρe vV = − , (1) ρe where ρe is the experimental density measured using a helium pycnometer model Pycnomatic ATC (Thermo Electron Corporation, Waltham, MA, US) and ρt is the theoretical density, calculated with the following equation: 1 ρt = , (2) W f + Wm ρ f ρm where ρm and Wm are the density and the weight content of the epoxy matrix, whereas ρ f and W f are the density and the weight content of natural fiber, respectively.
Table 1. List of composite laminates.
Code Stacking Sequence 1 Fabrics
Flax-AR [F]5 as received Flax-T [F]5 treated Jute-AR [J]5 as received Jute-T [J]5 treated F Hybrid-AR [F/J/F/J/F] as received F Hybrid-T [F/J/F/J/F] treated J Hybrid-AR [J/F/J/F/J] as received J Hybrid-T [J/F/J/F/J] treated 1 F = twill weave flax fabric; J = twill weave woven jute fabric.
Epoxy composite panels (25 cm 25 cm) were exposed in a climatic chamber model SC/KWT × 450 by Weiss (Germany) to salt-fog spray environment for up to 2 months, according to ASTM B 117 standard. After 30 and 60 days of salt-fog aging, specimens were cut through with a diamond saw from the center of each composite panel to perform the dynamic mechanical characterization. The evolution of the dynamic mechanical response of flax, jute and hybrid flax/jute laminates during aging exposition was done by comparing their mechanical properties with those of reference samples (i.e., unaged samples). The dynamic properties (i.e., storage modulus, loss modulus and tan δ) of each composite were evaluated through dynamic mechanical thermal tests (DMTAs), in accordance with ASTM D 4065 standard. The above characterization was carried out under three-point bending mode at constant frequency of 1 Hz, using a dynamic mechanical analyzer Metravib DMA+150 (Limonest, France). Three specimens (3 mm 46 mm) per laminate were tested in nitrogen atmosphere from × 25 ◦C to 140 ◦C at a heating rate of 3 ◦C/min. Polymers 2020, 12, 716 4 of 18
3. Results and Discussion
3.1. Effect of Treatment Figure1 shows the typical storage modulus (E’) trends of flax fiber-reinforced composites as a functionPolymers 2020 of, their 12, x FOR exposition PEER REVIEW time to salt-fog environment (i.e., 0 days or unaged, 30 days and 604 days).of 18 As is widely known [40,50], the storage modulus is typically associated with the material stiffness: it measuresAs is widely the material’s known capability [40,50], the to storestorage the modulus energy applied. is typically Two diassociatedfferent regions with the of the material storage modulusstiffness: it curve measures of fiber-reinforced the material’s polymerscapability byto varyingstore the temperature energy applied. were Two evidenced, different i.e.,regions the firstof onethe storage below glassmodulus transition curve of temperature fiber‐reinforced (Tg) polymers of the polymeric by varying matrix temperature and the secondwere evidenced, one above i.e., Tg, namedthe first glassy one below region glass and transition rubbery region, temperature respectively. (Tg) of In the the polymeric glassy region, matrix the and storage the second modulus one of fiber-reinforcedabove Tg, named composite glassy region materials and was rubbery high becauseregion, respectively. the components In the (i.e., glassy matrix region, and reinforcement) the storage weremodulus extremely of fiber immobilized,‐reinforced composite allowing a materials stiff behavior. was high Vice because versa, FRP the componentscomponents acquired(i.e., matrix mobility, and losingreinforcement) their close were packing extremely arrangement immobilized, in the allowing rubbery region,a stiff behavior. thus leading Vice toversa, decrease FRP ofcomponents the storage modulusacquired withmobility, temperature losing their [40]. close packing arrangement in the rubbery region, thus leading to decrease of the storage modulus with temperature [40].
(a) (b)
FigureFigure 1.1. Typical storage modulus trends trends of of flax flax composites composites in in each each aging aging condition: condition: (a ()a )Flax Flax-AR‐AR composites;composites; (b) Flax-TFlax‐T composites.
ByBy comparingcomparing Figure 11a,b,a,b, itit waswas notednoted that,that, inin thethe unagedunaged condition,condition, sodium bicarbonate bicarbonate treatmenttreatment allowed the composite stiffness stiffness to to increase increase in in the the glassy glassy region. region. In In particular, particular, the the storage storage modulusmodulus E’E’ at at 25 25 °CC of of Flax-T Flax‐T laminates laminates (i.e., (i.e., 5.0 5.0 ±0.1 0.1 GPa) GPa) was was about about 47% 47% higher higher than than that that of of Flax-AR Flax‐ ◦ ± laminatesAR laminates (i.e., (i.e., 3.4 3.40.2 ± 0.2 GPa). GPa). This This result result confirms confirms that that the the adhesionadhesion between flax flax fibers fibers and and epoxy epoxy ± matrixmatrix becomesbecomes strongest due to the the fiber fiber treatment, treatment, as as already already stated stated in in our our previous previous papers papers [46,48]. [46,48 ]. ItIt isis worthworth notingnoting that, during the exposition to to the the salt salt-fog‐fog environment, environment, Flax Flax-AR‐AR composites composites experiencedexperienced noticeablenoticeable decrements decrements of of E’ E’ in in the the glassy glassy region. region. In particular,In particular, laminates laminates aged aged for 1 monthfor 1 andmonth 2 months and 2 months showed showed storage storage modulus modulus values values 32% and 32% 49% and lower 49% thanlower the than reference the reference (i.e., unaged (i.e., laminates),unaged laminates), respectively. respectively. Vice versa, Vice a stable versa, dynamic a stable dynamic behavior behavior during the during exposition the exposition to the aggressive to the environmentaggressive environment was evidenced was evidenced by Flax-T composites.by Flax‐T composites. Indeed, the Indeed, soaking the ofsoaking flax fabrics of flax in fabrics NaHCO in 3 solutionNaHCO allowed3 solution to allowed obtain a compositeto obtain a laminate composite able laminate to better able retain to itsbetter storage retain dynamic its storage modulus dynamic below glassmodulus transition below temperature, glass transition i.e., temperature, in comparison i.e., to in unaged comparison samples to the unaged measured samples decrements the measured of E’ at 25decrements◦C were equal of E’ toat just 25 3%°C were and 8% equal after to 1 just month 3% and and 2 8% months after of1 aging,month respectively. and 2 months This of suggests aging, that,respectively. in addition This to suggests the quasi-static that, in mechanical addition to stability the quasi [32],‐static fiber mechanical treatment allows stability flax-reinforced [32], fiber epoxytreatment composites allows flax also‐reinforced to better retainepoxy their composites dynamic also behavior to better under retain salt-fog their dynamic exposition. behavior under salt‐fogLoss exposition. modulus E” represents the viscous response of the material (i.e., the material tendency to dissipateLoss modulus applied E’’ energy), represents whereas the viscous tan δ, response also named of the damping material or (i.e., loss the factor, material is defined tendency as to the lossdissipate modulus applied to storage energy), modulus whereas ratio tan δ (E”, also/E’). named A high damping tan δ value or loss is indicative factor, is defined of materials as the having loss amodulus great non-elastic to storage strainmodulus component ratio (E’’/E’). while A high a low tan value δ value indicates is indicative high of elasticity. materials This having means a great that, fornon fiber-reinforced‐elastic strain component composites, while an increase a low value of the indicates fiber/matrix high adhesion elasticity. results This means in the reductionthat, for fiber of tan‐ δreinforceddue to the composites, mobility decrement an increase of theof the molecular fiber/matrix chains adhesion surrounding results the in fiber the reduction/matrix interface of tan δ [51due,52 ]. to the mobility decrement of the molecular chains surrounding the fiber/matrix interface [51,52]. As a consequence, the lower the energy loss of a composite system compared to its storage capacity, the greater the tan δ value [42]. Figure 2 shows the typical tan δ versus temperature trends of flax fiber‐reinforced epoxy composites for each aging condition investigated. First of all, it is worth noting that two different tan δ peaks were evidenced for all samples (i.e., regardless the fiber treatment and aging exposition time). Several authors [38,53,54] stated that the first peak at lower temperatures is attributed to the glass
Polymers 2020, 12, 716 5 of 18
As a consequence, the lower the energy loss of a composite system compared to its storage capacity, the greater the tan δ value [42]. Figure2 shows the typical tan δ versus temperature trends of flax fiber-reinforced epoxy composites for each aging condition investigated. First of all, it is worth noting that two different tan δ peaks werePolymers evidenced 2020, 12, x FOR for all PEER samples REVIEW (i.e., regardless the fiber treatment and aging exposition time). Several5 of 18 authorsPolymers 2020 [38, 5312,, 54x FOR] stated PEER thatREVIEW the first peak at lower temperatures is attributed to the glass transition5 of 18 temperaturetransition temperature of the thermoset of the matrix,thermoset whereas matrix, the whereas second one the atsecond higher one temperatures at higher temperatures is related to the is micro-mechanicalrelatedtransition to thetemperature micro transition‐mechanical of the phenomena thermoset transition ofmatrix, thephenomena immobilized whereas of the the polymer second immobilized one layer at between higherpolymer temperatures fiber layer and between matrix, is i.e.,fiberrelated fiber–matrix and to matrix, the micro interphase.i.e.,‐ mechanicalfiber–matrix transition interphase. phenomena of the immobilized polymer layer between fiberThe and comparison matrix, i.e., of fiber–matrix Figure 2 2a,ba,b interphase. evidencedevidenced that, that, regardlessregardless the the aging aging condition,condition, Flax-AR Flax‐AR laminateslaminates The comparison of Figure 2a,b evidenced that, regardless the aging condition, Flax‐AR laminates showed higherhigher tan tanδ δ thanthan those those of Flax-T, of Flax thus‐T, confirming thus confirming the beneficial the beneficial effect of the effect sodium of the bicarbonate sodium showed higher tan δ than those of Flax‐T, thus confirming the beneficial effect of the sodium treatmentbicarbonate on treatment the fiber/ onmatrix the fiber/matrix adhesion, even adhesion, when flaxeven fiber-reinforced when flax fiber‐ compositesreinforced composites were exposed were to bicarbonate treatment on the fiber/matrix adhesion, even when flax fiber‐reinforced composites were aggressiveexposed to mediaaggressive such media as salt-fog such spray. as salt‐fog spray. exposed to aggressive media such as salt‐fog spray.
(a) (b) (a) (b) Figure 2. Typical tan δ trends of flax composites in each aging condition: (a) Flax‐AR composites; (b) FigureFigure 2. TypicalTypical tan tan δ δtrendstrends of of flax flax composites composites in ineach each aging aging condition: condition: (a) Flax (a)‐ Flax-ARAR composites; composites; (b) Flax‐T composites. (Flaxb) Flax-T‐T composites. composites. Furthermore, Figure 2a shows that, after salt‐fog aging, the tan δ peaks of Flax‐AR laminates Furthermore,Furthermore, Figure 22aa showsshows that,that, afterafter salt-fogsalt‐fog aging, the tan δ δpeakspeaks of of Flax Flax-AR‐AR laminates laminates (i.e., 1‐month and 2‐month aged samples) shifted to lower temperatures and became wider than the (i.e.,(i.e., 1-month1‐month andand 2-month2‐month aged samples) shifted shifted to to lower lower temperatures temperatures and and became became wider wider than than the the unagedunaged sample.sample. This experimental evidence evidence isis mainlymainly mainly explained explained explained by by by coupling coupling coupling the the the softening softening softening of of of matrix, matrix, matrix, duedue toto thethe composites’composites’ exposition toto the the humid humid environment, environment, and and the the low low arrangement arrangement of of the the flax flax flax microfibrilsmicrofibrilsmicrofibrils network, network,network, as as shown shown in Figureinin Figure3. FRP 3.3. FRP constituentsFRP constituentsconstituents gained gainedgained local mobility, locallocal mobility, mobility, losing theirlosing losing packing their their arrangementpackingpacking arrangementarrangement in the rubbery in the region,rubbery thus region,region, favoring thusthus favoringfavoring the shift the towardthe shift shift lowertoward toward temperature lower lower temperature temperature of tan δ of[ 40of tan]. tan δ δ [40].[40].
FigureFigure 3. SEMSEM micrograph of of Flax Flax-AR‐‐ARAR laminatelaminate surface surface after after 60 60 days days of of salt salt-fogsalt‐fog‐fog exposition. exposition.
From a morphological point of view, the Flax-AR laminate was characterized by low interfacial FromFrom aa morphologicalmorphological point of view, thethe FlaxFlax‐‐ARAR laminate laminate was was characterized characterized by by low low interfacial interfacial fiber–matrix adhesion, as can be seen from high tan δ peaks already shown in unaged conditions. This fiber–matrixfiber–matrix adhesion,adhesion, as can be seen fromfrom highhigh tantan δ δ peakspeaks alreadyalready shown shown in in unaged unaged conditions. conditions. implies that preferential pathways for water diffusion can be identified in the interface between fibers ThisThis impliesimplies thatthat preferential pathways forfor waterwater diffusiondiffusion can can be be identified identified in in the the interface interface between between fibersfibers andand matrix,matrix, thus stimulating thethe triggeringtriggering ofof prematurepremature waterwater‐induced‐induced degradationdegradation phenomenaphenomena inin thethe natural reinforced compositecomposite laminate.laminate. In In particular, particular, water water weakens weakens the the physical physical bondsbonds (i.e.,(i.e., vanvan der Waals interaction) withinwithin thethe flax/matrixflax/matrix interface,interface, asas wellwell as as causing causing the the hydrolysishydrolysis ofof chemicalchemical bonds at the interface,interface, thusthus leadingleading to to the the occurrence occurrence of of interfacial interfacial debonding debonding phenomenaphenomena [40].[40]. As evidenced in Table 2,2, thethe temperaturetemperature of of both both tan tan δ δ peakspeaks decreased decreased about about 10 10 °C °C afterafter 6060 daysdays ofof salt‐fog exposition (i.e.,(i.e., fromfrom 75.475.4 °C°C toto 65.565.5 °C, °C, and and from from 105.6 105.6 °C °C to to 95.2 95.2 °C, °C, for for the the
Polymers 2020, 12, 716 6 of 18 and matrix, thus stimulating the triggering of premature water-induced degradation phenomena in the natural reinforced composite laminate. In particular, water weakens the physical bonds (i.e., van der Waals interaction) within the flax/matrix interface, as well as causing the hydrolysis of chemical bonds at the interface, thus leading to the occurrence of interfacial debonding phenomena [40]. As evidenced in Table2, the temperature of both tan δ peaks decreased about 10 ◦C after 60 days of salt-fog exposition (i.e., from 75.4 ◦C to 65.5 ◦C, and from 105.6 ◦C to 95.2 ◦C, for the first and the second peak, respectively). As concerns the evolution of the peaks’ height, the first one varied from 0.57 to 1.15, whereas the second one varied from 0.32 to 1.13. Vice versa, no noticeable variation was evidenced on the tan δ trends of Flax-T composites when varying the aging exposition time (Figure2b). Both peaks remained slightly unchanged when varying the aging condition. In particular, after 60 days of salt-fog exposition, the temperature decrements were just equal to 0.7 ◦C (i.e., from 77.2 ◦C to 76.5 ◦C) and 2.4 ◦C (from 104.7 ◦C to 102.3 ◦C) for the first and the second peak, respectively (Table2). At the same time, the height of the first peak varied from 0.37 to 0.38, whereas the second one varied from 0.28 to 0.37. These results are in good agreement with those achieved from the quasi-static flexural characterization [32], confirming the improved mechanical stability shown by Flax-T laminates in comparison to Flax-AR ones. It is noteworthy that the second peak showed a greater sensitivity to aging in the salt-fog environment than the first one. This indicates that the water diffusion phenomena, which are considered responsible for the aging activation phenomena [55], are activated initially at the interface between the fiber and the matrix and then evolve towards the bulk of the matrix, thus causing its softening. The Flax-T laminate, being characterized by an improved interfacial adhesion, strongly limited the water absorption by inhibiting or extending over time the subsequent aging effect on the epoxy matrix.
Table 2. Damping properties of flax composites for each aging condition.
Flax-AR Flax-T Flax-AR Flax-T Aging Days First Peak T [◦C] First Peak Height 0 75.4 1.5 77.2 0.5 0.57 0.04 0.37 0.03 ± ± ± ± 30 70.2 0.4 76.2 0.7 0.99 0.01 0.37 0.00 ± ± ± ± 60 65.5 0.5 76.5 1.0 1.15 0.01 0.38 0.02 ± ± ± ± Second Peak T [◦C] Second Peak Height 0 105.6 1.3 104.7 1.3 0.32 0.02 0.28 0.01 ± ± ± ± 30 102.6 2.7 105.3 1.6 0.88 0.05 0.31 0.02 ± ± ± ± 60 95.2 1.3 102.3 2.3 1.13 0.12 0.37 0.05 ± ± ± ±
Figure4 shows the typical storage modulus versus temperature trends of jute laminates for each aging condition investigated. Regarding unaged samples, it is worth noting that, unlike flax laminates, fiber treatment did not increase the composite stiffness in the glassy region. In particular, epoxy laminates reinforced with untreated or raw jute fabrics (i.e., Jute-AR) showed E’ at room temperature (i.e., 4.3 0.1 GPa) about 47% higher than Jute-T laminates (i.e., 2.9 0.3 GPa). This experimental ± ± result confirms that the adverse effect of the sodium bicarbonate treatment on the adhesion between jute fibers and epoxy resin leads to decreasing the dynamic modulus of the resulting composites [48]. As a consequence, Jute-T laminates experienced greater decrements of storage modulus in the glassy region due to the salt-fog exposition in comparison to their counterparts (i.e., Jute-AR laminates). In more detail, the storage modulus values at room temperature of Jute-AR-1m and Jute-AR-2m laminates were 10% and 18% lower than that of the unaged laminates, respectively. On the contrary, Jute-T composites evidenced decrements in the storage modulus at room temperature equal to 25% and 40%, after 1 month and 2 months of aging exposition, respectively. Polymers 2020, 12, x FOR PEER REVIEW 7 of 18 PolymersPolymers2020 2020, 12, 12, 716, x FOR PEER REVIEW 7 of7 18 of 18
(a(a) ) (b)
FigureFigureFigure 4.4. 4. Typical Typical storage storage modulus modulus trends trendstrends of of jute jute compositescomposites inin in each each aging aging condition: condition: ((aa ())a Jute)Jute Jute-AR‐‐ARAR composites;composites;composites; (b (b) ) Jute-TJute Jute‐T‐T composites. composites.
ByByBy observingobserving observing the the typical typical tan tan δ δ δ versusversus temperature temperature trendstrends trends of of jute jute composites composites atat at varyingvarying varying agingaging aging conditionsconditionsconditions (Figure(Figure (Figure5 ),5), 5), it isit it evidencedis is evidenced evidenced that, that,that, regardless regardlessregardless of fiber ofof fiberfiber treatment, treatment, both both tan δ tantanpeaks δ δ peakspeaks became becamebecame higher, alsohigher,higher, shifting also also shifting to shifting lower to temperatures,to lower lower temperatures, temperatures, when increasing whenwhen increasingincreasing the salt-fog the expositionsalt‐fog exposition time. Since time.time. the SinceSince damping thethe behaviordampingdamping of behaviorbehavior fiber-reinforced ofof fiberfiber composites‐reinforced‐reinforced iscompositescomposites greatly influenced isis greatly by influenced the shear stressby thethe concentrations shearshear stressstress at theconcentrationsconcentrations fiber–matrix at at interfaces, the the fiber–matrix fiber–matrix as well interfaces, interfaces, as by the asas viscoelastic wellwell asas byby thethe energy viscoelastic dissipation energy within dissipationdissipation the polymeric withinwithin matrixthethe polymeric polymeric [51], these matrix matrix results [51], [51], are these these attributed results results both areare attributedattributed to the matrix bothboth softening toto the matrix and tosoftening the fiber–matrix andand toto thethe interface fiber–fiber– degradationmatrixmatrix interface interface phenomena, degradation degradation due tophenomena, phenomena, the composites’ duedue toto exposition thethe composites’composites’ to salt-fog. exposition As already toto saltsalt stated,‐‐fog.fog. As noAs already beneficialalready estated,ffstated,ect on no no fiber–matrix beneficial beneficial effect compatibilityeffect onon fiber–matrixfiber–matrix was achieved compatibilitycompatibility after jute was fiber achieved treatment. after Consequently, jutejute fiberfiber treatment.treatment. Figure 5 clearlyConsequently,Consequently, shows that, Figure Figure regardless 5 5 clearly clearly the shows shows aging that, that, time regardlessregardless exposition, thethe tan agingagingδ values time exposition, of Jute-AR tan laminatestan δ δ valuesvalues were ofof JuteJute lower‐‐ thanARAR laminates those laminates of Jute-T were were in lower lower the whole than than temperature thosethose ofof JuteJute range‐‐TT inin the becausethe wholewhole they temperature were characterized range becausebecause by higher theythey mobility werewere ofcharacterizedcharacterized the molecular by by chains higher higher both mobility mobility in the ofof matrix thethe molecularmolecular bulk and chainschains surrounding both in the the fiber matrix/matrix bulk interface. andand surroundingsurrounding thethe fiber/matrixThis fiber/matrix result interface. indicates interface. that composites reinforced with untreated Jute fibers (i.e., Jute-AR) are characterizedThisThis result result by indicates betterindicates fiber–matrix thatthat compositescomposites interfacial reinforcedreinforced adhesion withwith than untreated surface Jute treated fibers ones (i.e.,(i.e., (i.e., JuteJute Jute-T)‐‐AR)AR) areare [47 ]. characterized by better fiber–matrix interfacial adhesion than surface treated ones (i.e., Jute‐T) [56]. Duecharacterized to the soaking by better of jute fiber–matrix fibers in theinterfacial NaHCO adhesion3 solution, than a surface reduction treated of lignin ones and(i.e., hemicellulose Jute‐T) [56]. Due to the soaking of jute fibers in the NaHCO3 solution, a reduction of lignin and hemicellulose componentsDue to the soaking from the of fiberjute fibers bulk occurredin the NaHCO [44], thus3 solution, favoring a reduction the triggering of lignin of damage and hemicellulose phenomena components from the fiber bulk occurred [44], thus favoring the triggering of damage phenomena at atcomponents the interface. from Indeed, the fiber Jute-T bulk occurred laminates [44], showed thus favoring a larger decrementthe triggering in theirof damage dynamic phenomena mechanical at the interface. Indeed, Jute‐T laminates showed a larger decrement in their dynamic mechanical performancesthe interface. thanIndeed, Jute-AR Jute‐T laminates laminates during showed the aginga larger exposition, decrement due in to their their dynamic higher sensitivity mechanical to a performances than Jute‐AR laminates during the aging exposition, due to their higher sensitivity to water-basedperformances environment. than Jute‐AR laminates during the aging exposition, due to their higher sensitivity to a awater water‐based‐based environment. environment.
(a) (b) (a) (b) FigureFigure 5. 5.Typical Typical tantan δ δ trendstrends of of jute jute composites composites in ineach each aging aging condition: condition: (a) Jute (a)‐AR Jute-AR composites; composites; (b) Figure 5. Typical tan δ trends of jute composites in each aging condition: (a) Jute‐AR composites; (b) (bJute) Jute-T‐T composites. composites. Jute‐T composites. InIn detail, detail, it it was was found found that that Jute-TJute‐T laminateslaminates showed greater greater variations variations of of both both tan tan δ peaksδ peaks with with increasingincreasingIn detail, exposition exposition it was found time time to that salt-fogto Jutesalt‐fogT environment. laminates environment. showed In particular, In greaterparticular, aftervariations after 60 days 60of inbothdays the tan climaticin δ thepeaks climatic chamber, with theincreasing temperature exposition of the firsttime peak to salt decreased‐fog environment. from 82.0 CIn to particular, 76.3 C (i.e., after∆T =605.8 daysC) andin the from climatic 81.5 C ◦ ◦ ◦ ◦
Polymers 2020, 12, 716 8 of 18
to 72.0 ◦C (i.e., ∆T = 9.5 ◦C) for Jute-AR and Jute-T laminates, respectively (Table3). With regard to the second tan δ peak, its temperature decrements were found equal to 3.3 ◦C (i.e., from 108.3 ◦C to 105.0 ◦C) and 4.1 ◦C (i.e., from 109.0 ◦C to 104.9 ◦C) for Jute-AR and Jute-T laminates, respectively. At the same time, Jute-T laminates showed increments from 0.33 to 0.62 and from 0.23 to 0.51 in the heights of the first and second tan δ peaks, respectively. Vice versa, epoxy laminates reinforced with jute untreated fibers (i.e., Jute-AR) showed lower height increments of both peaks (i.e., from 0.22 to 0.31 for the first peak and from 0.16 to 0.24 for the second one). These results are ascribed to the weaker fiber–matrix interfacial adhesion evidenced by Jute-T laminates in comparison to Jute-AR ones [32,33], which triggers water diffusion and, consequently, the composite aging after a short time of exposition to salt-fog environment.
Table 3. Damping properties of jute composites for each aging condition.
Jute-AR Jute-T Jute-AR Jute-T Aging Days First Peak T [◦C] First Peak Height 0 82.0 0.1 81.5 0.7 0.22 0.01 0.33 0.01 ± ± ± ± 30 78.6 0.6 76.2 0.8 0.26 0.02 0.51 0.00 ± ± ± ± 60 76.3 1.1 72.0 1.6 0.31 0.05 0.62 0.01 ± ± ± ± Second Peak T [◦C] Second Peak Height 0 108.3 1.0 109.0 4.6 0.16 0.01 0.23 0.07 ± ± ± ± 30 106.7 2.1 108.0 2.4 0.22 0.01 0.46 0.05 ± ± ± ± 60 105.0 1.9 104.9 2.5 0.24 0.04 0.51 0.08 ± ± ± ±
3.2. Effect of Stacking Sequence The typical trends of the damping properties (i.e., storage modulus E’ and tan δ) versus temperature of F Hybrid laminates are shown in Figure6 for each aging condition. By observing this figure, it can be noted that, even for hybrid stacking sequence, NaHCO3 treatment allowed to modify the dynamic properties of the resulting composites. As shown in Figure6a,b, F Hybrid-T laminates evidenced higher stiffness in the glassy region than their counterparts (i.e., F Hybrid-AR), regardless the aging time exposition. In particular, the average storage moduli E’ measured at 25 ◦C of laminates reinforced with treated fibers (i.e., F Hybrid-T) were about 22%, 44% and 64% higher than those of F Hybrid-AR, after 0, 30 and 60 days in salt-fog environment, respectively. Accordingly, the comparison of Figure6c,d evidenced that F Hybrid-AR unaged laminates showed higher tan δ values than Flax-T unaged ones, in the whole temperature range. In addition, the exposition to salt-fog clearly modified the tan δ curves of F Hybrid-AR laminates, since both tan δ peaks became higher and shifted to lower temperatures when increasing the exposition time. After 60 days of salt-fog exposition, the tan δ peak temperatures decreased about 8 ◦C (i.e., from 75.7 ◦C to 68.0 ◦C) and 5 ◦C (i.e., from 107.0 ◦C to 101.9 ◦C), for the first and the second one, respectively. As regards the peaks’ heights, the first peak grew from 0.48 to 0.69, whereas the second one grew from 0.42 to 0.87 (Table4). Vice versa, the typical tan δ trends of F Hybrid-T composites remained almost unchanged when varying the exposition time to salt-fog environment, as shown in Figure6d, i.e., no noticeable variation was observed by comparing tan δ peaks of unaged and salt-fog aged F Hybrid-T laminates. Both peaks remained unchanged when varying the aging condition. In particular, after 60 days of salt-fog exposition, the temperature decrements for both peaks were just equal to 1.8 ◦C (i.e., from 79.4 ◦C to 77.6 ◦C and from 104.3 ◦C to 102.5 ◦C for the first and the second one, respectively). At the same time, the height of the first peak varied from 0.24 to 0.28, whereas the second one varied from 0.20 to 0.28. Polymers 2020, 12, 716 9 of 18 Polymers 2020, 12, x FOR PEER REVIEW 9 of 18
(a) (b)
(c) (d)
FigureFigure 6. 6. (a)(‐a(,bb)) Typical Typical storage storage modulus modulus and and (c) (‐c(,d)) tan tan δ δtrendstrends of of F F-Hybrid‐Hybrid composites composites in ineach each agingaging condition. condition.
Table 4. Damping properties of F Hybrid composites for each aging condition. Table 4. Damping properties of F Hybrid composites for each aging condition. F Hybrid‐AR F Hybrid‐T F Hybrid‐AR F Hybrid‐T Aging days F Hybrid-AR F Hybrid-T F Hybrid-AR F Hybrid-T Aging Days First peak T [°C] First peak height 0 75.7First ± 0.3 Peak T79.4 [◦C] ± 0.8 0.48 ± 0.03 First0.24 Peak ± 0.01 height 30 73.0 ± 1.5 81.3 ± 0.6 0.61 ± 0.04 0.25 ± 0.01 0 75.7 0.3 79.4 0.8 0.48 0.03 0.24 0.01 ± ± ± ± 3060 73.068.0 1.5± 0.9 77.6 81.3 ± 3.50.6 0.69 ± 0.61 0.01 0.040.28 ± 0.02 0.25 0.01 ± ± ± ± 60 68.0 Second0.9 peak T 77.6 [°C] 3.5Second 0.69 peak0.01 height 0.28 0.02 0 107.0± ± 0.7 102.2 ±± 2.4 0.42 ± 0.04± 0.20 ± 0.03 ± Second Peak T [ C] Second Peak Height 30 102.2 ± 2.4 99.9◦ ± 1.4 0.68 ± 0.02 0.23 ± 0.01 060 107.0101.9 0.7± 2.9 102.5 102.2 ± 4.72.4 0.87 ± 0.42 0.01 0.040.28 ± 0.01 0.20 0.03 ± ± ± ± 30 102.2 2.4 99.9 1.4 0.68 0.02 0.23 0.01 All these results can be ascribed± to the different± interactions of± the two natural fibers± with the 60 101.9 2.9 102.5 4.7 0.87 0.01 0.28 0.01 sodium bicarbonate treatment. If,± on one hand, ±the bicarbonate ±treatment was widely± proven beneficial to the adhesion between flax fibers and epoxy matrix, on the other hand, it was equally evidentAll that these the results same treatment can be ascribed weakened to the the di jutefferent fiber–epoxy interactions matrix of the interface. two natural Consequently, fibers with it is the reasonablesodium bicarbonate to consider treatment. that F Hybrid If, on‐ oneT laminates hand, the showed bicarbonate better treatment damping was properties widely provenin comparison beneficial toto thetheir adhesion untreated between counterparts flax fibers (i.e., and F epoxyHybrid matrix,‐AR) because on the otherthe stacking hand, it sequence was equally of evidentF Hybrid that laminatesthe same treatmentis characterized weakened by more the juteflax‐ fiber–epoxyreinforced laminae matrix interface.than jute ones Consequently, (i.e., three itversus is reasonable two). Moreover,to consider these that hybrid F Hybrid-T laminates laminates present showed flax‐reinforced better damping laminae propertiesas external in layers. comparison The bending to their performancesuntreated counterparts of the composite (i.e., F laminates Hybrid-AR) are heavily because influenced the stacking by these sequence external of laminae F Hybrid that laminates suffer theis characterized higher longitudinal by more stresses, flax-reinforced unlike the laminae central than laminae jute ones where (i.e., the three neutral versus axis two). is Moreover,located. Furthermore,these hybrid laminatesit can be evidenced present flax-reinforced that F Hybrid laminae‐T laminates as external showed layers. better The damping bending properties performances in comparisonof the composite to their laminates untreated are counterparts heavily influenced (i.e., F Hybrid by these‐AR) external because laminae the stacking that su sequenceffer the higherof F Hybridlongitudinal laminates stresses, is characterized unlike the central by a larger laminae amount where of the flax neutral‐reinforced axis islaminae located. than Furthermore, jute ones (i.e., it can threebe evidenced reinforced that laminae F Hybrid-T versus laminatestwo). showed better damping properties in comparison to their untreatedOn the counterparts contrary, J Hybrid (i.e., F laminates Hybrid-AR) have because three epoxy the stacking‐based laminae sequence reinforced of F Hybrid with laminatesjute fibers is versus two reinforced with flax ones in their stacking sequence. In addition, the external laminae
Polymers 2020, 12, 716 10 of 18 characterized by a larger amount of flax-reinforced laminae than jute ones (i.e., three reinforced laminae versus two). On the contrary, J Hybrid laminates have three epoxy-based laminae reinforced with jute fibersPolymers versus 2020, 12, two x FOR reinforced PEER REVIEW with flax ones in their stacking sequence. In addition, the external10 of 18 laminae supporting the highest longitudinal stresses are reinforced with jute fibers. Consequently, thesupporting detrimental the ehighestffect of thelongitudinal treatment onstresses jute fiber–epoxy are reinforced matrix with adhesion, jute fibers. in addition Consequently, to worsening the thedetrimental damping effect properties of the treatment of J Hybrid-T on jute laminates fiber–epoxy in comparison matrix adhesion, to the laminatesin addition reinforced to worsening with the as receiveddamping fabrics properties (i.e., J Hybrid-AR)of J Hybrid‐ inT thelaminates unaged in condition comparison [32], reduced to the laminates their service reinforced life when with exposed as received fabrics (i.e., J Hybrid‐AR) in the unaged condition [32], reduced their service life when to salt-fog. exposed to salt‐fog. Indeed, J Hybrid-T laminates showed the greater decrement of the storage modulus E’ in the Indeed, J Hybrid‐T laminates showed the greater decrement of the storage modulus E’ in the glassy region with the exposition time to salt-fog in comparison to J Hybrid-AR laminates, as evidenced glassy region with the exposition time to salt‐fog in comparison to J Hybrid‐AR laminates, as by comparing Figure7a,b. In particular, it was found that E’ measured at room temperature of J evidenced by comparing Figure 7a,b. In particular, it was found that E’ measured at room Hybrid-T laminates after 1 month and 2 months under salt-fog environment was 23% and 37% lower temperature of J Hybrid‐T laminates after 1 month and 2 months under salt‐fog environment was than unaged laminates, respectively. On the other hand, J Hybrid-AR composites better retained their 23% and 37% lower than unaged laminates, respectively. On the other hand, J Hybrid‐AR composites dynamic stiffness, showing a reduction of the storage modulus at room temperature equal to 12% and better retained their dynamic stiffness, showing a reduction of the storage modulus at room 24% after 1 month and 2 months of aging exposition, respectively. temperature equal to 12% and 24% after 1 month and 2 months of aging exposition, respectively.
(a) (b)
(c) (d)
FigureFigure 7. 7. (a()a‐(,b)) Typical Typical storage storage modulus modulus and and (c)‐( dc,)d tan) tan δ trendsδ trends of J‐ ofHybrid J-Hybrid composites composites in each in aging each agingcondition. condition.
ConcerningConcerning the eeffectffect of salt-fogsalt‐fog exposition on tan δ δ trends,trends, Figure Figure 77cc,d and evidences 7d evidences that both that tanbothδ peakstan δ peaks became became higher higher and shifted and shifted to lower to lower temperatures temperatures when when increasing increasing the salt-fog the salt exposition‐fog exposition time, fortime, laminates for laminates reinforced reinforced with with treated treated or as or received as received fabrics. fabrics. However, However, it isit is worth worth noting noting that that tan tanδ valuesδ values of Jof Hybrid-AR J Hybrid‐AR laminates laminates were were lower lower than than those those of J Hybrid-Tof J Hybrid in‐ theT in whole the whole temperature temperature range, regardlessrange, regardless the aging the exposition aging exposition time. In time. accordance In accordance with the with evolution the evolution of the storageof the storage modulus, modulus, hybrid laminateshybrid laminates reinforced reinforced with treated with treated fabrics fabrics (i.e., J Hybrid-T)(i.e., J Hybrid showed‐T) showed greater greater variations variations of both of both tan δ tan δ peaks with salt‐fog exposition time in comparison to J Hybrid‐AR laminates. Table 5 evidences that the temperature of the first peak slightly decreased 5.4 °C (i.e., from 79.9 °C to 74.5 °C) and 10.1 °C (i.e., from 81.9 °C to 71.8 °C) for J Hybrid‐T and J Hybrid‐AR laminates after 60 days under salt‐ fog, respectively. The second tan δ peak decreased from 109.6 °C to 99.9 °C (i.e., T = 9.7 °C) and 106.8 °C to 103.6 °C (i.e., T = 3.2 °C) for J Hybrid‐T laminates and J Hybrid‐AR, respectively.
Polymers 2020, 12, 716 11 of 18 peaks with salt-fog exposition time in comparison to J Hybrid-AR laminates. Table5 evidences that the temperature of the first peak slightly decreased 5.4 ◦C (i.e., from 79.9 ◦C to 74.5 ◦C) and 10.1 ◦C (i.e., from 81.9 ◦C to 71.8 ◦C) for J Hybrid-T and J Hybrid-AR laminates after 60 days under salt-fog, respectively. The second tan δ peak decreased from 109.6 ◦C to 99.9 ◦C (i.e., ∆T = 9.7 ◦C) and 106.8 ◦C to 103.6 ◦C (i.e., ∆T = 3.2 ◦C) for J Hybrid-T laminates and J Hybrid-AR, respectively.
Table 5. Damping properties of J Hybrid composites for each aging condition.
J Hybrid-AR J Hybrid-T J Hybrid-AR J Hybrid-T Aging Days First Peak T [◦C] First Peak Height 0 81.9 0.9 79.9 0.2 0.33 0.02 0.43 0.00 ± ± ± ± 30 80.5 2.0 80.5 3.4 0.38 0.02 0.56 0.12 ± ± ± ± 60 71.8 0.1 74.5 5.5 0.46 0.02 0.63 0.04 ± ± ± ± Second Peak T [◦C] Second Peak Height 0 106.8 1.1 109.6 1.1 0.18 0.03 0.38 0.04 ± ± ± ± 30 105.9 5.0 100.1 3.0 0.27 0.07 0.62 0.01 ± ± ± ± 60 103.6 4.2 99.9 2.7 0.33 0.02 0.86 0.09 ± ± ± ±
At the same time, J Hybrid-T laminates increased the heights of first and second tan δ peaks from 0.43 to 0.63 and from 0.38 to 0.86 after 60 aging days, respectively. Vice versa, hybrid laminates reinforced with untreated fabrics (i.e., J Hybrid-AR) showed reduced height increments of both peaks (i.e., from 0.33 to 0.46 for the first peak and from 0.18 to 0.33 for the second one). This phenomenon can be related to the effect induced by the sodium bicarbonate treatment that, as reported in [48], leads to a significant enhancement of the quasi-static performances of flax-reinforced composites and, conversely, a reduction in strength and stiffness of jute-based composites.
3.3. Damping Properties Versus Moisture Content Useful information can be extrapolated by analyzing the correlation between the damping properties variation and the progressive water absorption evidenced by the NFRPs during their exposition to the salt-fog condition. This approach allows to discriminate how mechanical parameters are more affected by degradative phenomena. In such a context, Table6 reports the water uptake values evidenced by all the compared laminates after 30 days and 60 days under salt-fog condition, respectively.
Table 6. Water absorption percentage of all laminates in each aging condition.
Laminate 30 Days 60 Days Flax-AR 13.7 0.5 14.2 0.5 ± ± Flax-T 10.1 0.5 10.5 0.5 ± ± Jute-AR 10.7 0.3 11.7 0.6 ± ± Jute-T 12.9 0.3 13.6 0.7 ± ± F Hybrid-AR 12.1 0.7 12.9 0.5 ± ± F Hybrid-T 10.5 0.5 11.1 0.5 ± ± J Hybrid-AR 11.5 0.6 12.4 0.5 ± ± J Hybrid-T 12.4 0.3 13.1 0.7 ± ±
It is worth noting that all samples evidenced a relevant hydrophilic behavior, showing weight gain values in the range 10–13% after 30 days of salt-fog exposition. Moreover, it was found that Flax-AR and Jute-T laminates were the most sensitive laminates to wet environments. This indicates that the water uptake is not related to the water sorption capability of natural fibers or epoxy matrix, but it is mainly related to competing fiber–matrix phenomena that synergistically play an important role in the degradative process of these NFRP materials. Polymers 2020, 12, 716 12 of 18
In order to relate the dynamic mechanical performance modification with environmental aging in wet conditions, the relationships between the variation of damping properties of all the resulting laminates and their moisture content are shown in Figures8–10. In particular, the variation of each damping property was calculated as follows:
∆P = Pt/P0, (3) where Pt is the value of the damping property of the laminate under salt-fog condition for a specific time t (i.e., 1 or 2 months) and P0 is the value of the same property at the beginning of the aging campaign (i.e., for the unaged condition). PolymersIn 2020 particular,, 12, x FOR Figure PEER 8REVIEW shows that moisture had a significant influence on the storage modulus12 of E’18 measured at 25 ◦C of NFRP laminates. It was clearly found a linear relationship between the storage modulus variation variation (indicated (indicated as as SMV SMV index) index) and and the the water water uptake. uptake. Similar Similar to what to what was was observed observed by Wangby Wang and and Petru Petru [40], [40 a], clear a clear decrease decrease of of the the dynamic dynamic stiffness stiffness was was found found with with increasing increasing the composites’ moisture content. Due Due to to the the polysaccharide polysaccharide content content of of natural natural fibers fibers and the voids and cracks in the epoxy matrix bulk, flax flax and jute fibers fibers tended to absorb a great amount of water when composites werewere exposed exposed to humidto humid environments environments such assuch a marine as a onemarine [10,23 one]. Hence, [10,23]. the Hence, penetration the penetration of water molecules and +ions (i.e., Na+ and Cl−) within the composite structure led to of water molecules and ions (i.e., Na and Cl−) within the composite structure led to softening both softeningmatrix and both natural matrix reinforcing and natural fibers, reinforcing as well asfibers, chemically as well and as chemically physically and degrading physically the degrading inherently theweak inherently fiber/matrix weak interface. fiber/matrix interface. Furthermore, it it is is worth worth noting noting that that composites composites having having the strongest the strongest fiber–matrix fiber–matrix adhesion adhesion (i.e., Flax(i.e.,‐T Flax-T and andF Hybrid F Hybrid-T‐T laminates) laminates) [48] [ 48were] were characterized characterized by by lower lower moisture moisture absorptions absorptions and, and, consequently, more contained stiffness reductions (as visible by the positioning of purple dots in the graph of Figure 88).). On the contrary, thethe highesthighest valuesvalues ofof moisturemoisture absorptionabsorption andand storagestorage modulusmodulus reductions werewere shown shown by by composites composites with with weak weak fiber–matrix fiber–matrix adhesion adhesion (i.e., Flax-AR, (i.e., Flax Jute-T,‐AR, J Hybrid-T,Jute‐T, J HybridF Hybrid-AR).‐T, F Hybrid These‐AR). results These can results be explained, can be explained, taking into taking account into account the different the different interactions interactions of the ofinvestigated the investigated natural natural fibers (i.e.,fibers flax (i.e., and flax jute) and with jute) the with sodium the sodium bicarbonate bicarbonate treatment. treatment. As shown As shown in our inprevious our previous paper [ 48paper], bicarbonate [48], bicarbonate treatment treatment softens jute softens fibers, jute due tofibers, their due higher to amounttheir higher of lignin amount (i.e., upof ligninto 25–27%) (i.e., inup comparison to 25–27%) toin flax comparison fibers (i.e., to up flax to fibers 5%) [1 (i.e.,,56,57 up]. Conversely,to 5%) [1,57,58]. this mildly Conversely, alkaline this treatment mildly alkalineis able to treatment remove the is impurities able to remove from the the surface impurities of flax fibersfrom withoutthe surface any chemicalof flax fibers interaction without with any the chemicalfiber bulk interaction because of the with lower the lignin fiber contentbulk because and the of more the lower compact lignin structure, content compared and the to more jute fibers compact [58]. structure,These different compared interactions to jute fibers allow [59]. composites These different reinforced interactions with treated allow flax composites fibers to havereinforced a stronger with treatedfiber–matrix flax fibers adhesion to have in comparison a stronger to fiber–matrix those reinforced adhesion with raw in comparison fibers. Conversely, to those a slightreinforced worsening with rawof adhesion fibers. Conversely, between jute a slight fibers worsening and epoxy resinof adhesion is achieved between after jute sodium fibers bicarbonate and epoxy treatment.resin is achieved after sodium bicarbonate treatment.
Figure 8. Relationship between moisture content and the variation of storage modulus E’ measured at Figure 8. Relationship between moisture content and the variation of storage modulus E’ measured room temperature. at room temperature.
In accordance with the trend of the dynamic storage modulus, the height of loss factor (i.e., tan δ) peaks increased with the moisture content, as shown in Figure 9 and Figure 10. It is well known that the loss factor versus temperature trend of FRP materials is influenced by the incorporation of fibers in the polymeric matrix, due to the energy dissipation in the matrix bulk and to the shear stress concentrations at the fiber–matrix interfaces [51]. In particular, composites with a weak fiber–matrix interface show high and large loss factor peaks because they tend to dissipate more energy in comparison to FRPs having the strongest fiber–matrix adhesion [54,60]. Moreover, when composites are exposed to a humid environment, the value of loss factor peaks increases with the exposition time to humid environments and, consequently, moisture content [40]. This detrimental effect is more pronounced for composites characterized by a weak fiber–matrix interface because they dramatically suffer mechanical performance degradation when exposed to humid environments during their service life [12,13].
Polymers 2020, 12, 716 13 of 18
In accordance with the trend of the dynamic storage modulus, the height of loss factor (i.e., tan δ) peaks increased with the moisture content, as shown in Figures9 and 10. It is well known that the loss factor versus temperature trend of FRP materials is influenced by the incorporation of fibers in the polymeric matrix, due to the energy dissipation in the matrix bulk and to the shear stress concentrations at the fiber–matrix interfaces [51]. In particular, composites with a weak fiber–matrix interface show high and large loss factor peaks because they tend to dissipate more energy in comparison to FRPs having the strongest fiber–matrix adhesion [54,59]. Moreover, when composites are exposed to a humid environment, the value of loss factor peaks increases with the exposition time to humid environments and, consequently, moisture content [40]. This detrimental effect is more pronounced for composites characterized by a weak fiber–matrix interface because they dramatically suffer mechanical performance degradation when exposed to humid environments duringPolymers 2020 their, 12 service, x FOR PEER life [ 12REVIEW,13]. 13 of 18
Figure 9. Relationship between moisture content and the variation of first loss factor peak (LFV index). Figure 9. Relationship between moisture content and the variation of first loss factor peak1 (LFV1 index). By observing the trend of LFV1 (i.e., loss factor variation for the first peak) index as a function of the moisture content, it was possible to identify two different stages of the degradative process By observing the trend of LFV1 (i.e., loss factor variation for the first peak) index as a function of (Figure9). The first one (i.e., Stage 1) occurring at low water uptake values, was characterized by the moisture content, it was possible to identify two different stages of the degradative process limited variation of the loss factor for the first peak. This could be ascribed to a water uptake not (Figure 9). The first one (i.e., Stage 1) occurring at low water uptake values, was characterized by relevantlimited variation enough to of modify the loss the factor mobility for the of thefirst polymer peak. This chains could in the be matrixascribed bulk. to a On water the otheruptake hand, not whenrelevant the enough water absorption to modify the phenomenon mobility of became the polymer significant, chains a in relevant the matrix enhancement bulk. On the of other LFV1 hand,index was highlighted, as evidenced by the increased slope of the curve trend (i.e., Stage 2). Water molecules when the water absorption phenomenon became significant, a relevant enhancement of LFV1 index arewas able highlighted, to diffuse as inside evidenced the laminate by the structure, increased thus slope triggering of the softeningcurve trend phenomena (i.e., Stage of the2). matrixWater with,molecules as a consequence, are able to diffuse greater inside LFV1 theindex laminate values structure, occurring. thus Indeed, triggering samples softening evidencing phenomena the highest of water absorption showed LFV1 index values higher than 1.5. the matrix with, as a consequence, greater LFV1 index values occurring. Indeed, samples evidencing As evidenced in Figure9, the water uptake level equal to 12% can be considered as a threshold the highest water absorption showed LFV1 index values higher than 1.5. transitionAs evidenced value between in Figure the 9, two the abovementioned water uptake level stages equal for to these 12% natural can be compositeconsidered laminates. as a threshold transitionSimilar value considerations between the were two drawn abovementioned by observing stages the LFV for2 these(i.e., lossnatural factor composite variation laminates. for the second peak) index versus water uptake trend shown in Figure 10. It was of upmost importance to evidence that the second peak variation was more affected by water uptake in comparison to the first peak. Even in this case, two different degradative stages were identified at low and high water uptake values, respectively. Noticeable variations of the second peak height were evidenced already in the first stage, i.e., LFV2 values equal to about 1.4 for water uptake lower than 12%. These experimental results confirmed that the fiber–matrix adhesion weakening was strongly correlated with the water absorption experienced by the laminate in the beginning phase of the salt-fog exposition.
Figure 10. Relationship between moisture content and the variation of second loss factor peak (LFV2 index).
Similar considerations were drawn by observing the LFV2 (i.e., loss factor variation for the second peak) index versus water uptake trend shown in Figure 10. It was of upmost importance to evidence that the second peak variation was more affected by water uptake in comparison to the first peak. Even in this case, two different degradative stages were identified at low and high water uptake values, respectively. Noticeable variations of the second peak height were evidenced already in the
Polymers 2020, 12, x FOR PEER REVIEW 13 of 18
Figure 9. Relationship between moisture content and the variation of first loss factor peak (LFV1 index).
By observing the trend of LFV1 (i.e., loss factor variation for the first peak) index as a function of the moisture content, it was possible to identify two different stages of the degradative process (Figure 9). The first one (i.e., Stage 1) occurring at low water uptake values, was characterized by limited variation of the loss factor for the first peak. This could be ascribed to a water uptake not relevant enough to modify the mobility of the polymer chains in the matrix bulk. On the other hand, when the water absorption phenomenon became significant, a relevant enhancement of LFV1 index was highlighted, as evidenced by the increased slope of the curve trend (i.e., Stage 2). Water molecules are able to diffuse inside the laminate structure, thus triggering softening phenomena of the matrix with, as a consequence, greater LFV1 index values occurring. Indeed, samples evidencing the highest water absorption showed LFV1 index values higher than 1.5. Polymers 2020, 12, 716 14 of 18 As evidenced in Figure 9, the water uptake level equal to 12% can be considered as a threshold transition value between the two abovementioned stages for these natural composite laminates.
Figure 10. Relationship between moisture content and the variation of second loss factor peak Figure 10. Relationship between moisture content and the variation of second loss factor peak (LFV2 (LFV2 index). index). Therefore, this stage can be considered a discriminating factor concerning the micro-mechanical transitionSimilar phenomena considerations of were the immobilizeddrawn by observing polymer the layer LFV2 between (i.e., loss fiberfactor and variation matrix for (i.e., the fiber–matrixsecond peak) interphase). index versus water uptake trend shown in Figure 10. It was of upmost importance to evidenceOn this that basis, the second it is reasonable peak variation to consider was more that affected the water by di waterffusion uptake inside in the comparison composite to structure the first mainlypeak. Even depended in this oncase, a weak two different fiber–matrix degradative adhesion stages that, in were turn, identified represented at low the and main high factor water activating uptake degradativevalues, respectively. phenomena. Noticeable The transition variations between of the second the two peak degradative height were stages evidenced was identified, already also in the in this case, at water uptake values equal to 12%. Beyond this threshold limit, the LFV2 index became higher than 2 (i.e., up to 3.5 for Flax-AR samples, for instance), thus indicating a serious loss of the adhesion between the main components of the composite (i.e., fiber and matrix). Due to the above reasons, composites having the weakest fiber–matrix adhesion, such as Flax-AR and Jute-T, evidenced the highest increments in loss factor peaks as well as the greatest moisture absorption during their exposition to the salt-fog environment. On the contrary, the beneficial effect of the NaHCO3 treatment on the adhesion between flax fibers and epoxy resin allowed composites such as Flax-T and F-Hybrid T to reduce their moisture absorption and, as a consequence, to better retain their damping peaks. Similarly, composites with inherently good fiber–matrix adhesion, such as Jute-AR and J Hybrid-AR, showed low moisture adsorption and slight variations in both loss factor peaks, also after long exposition time in the salt-fog environment. With the aim to better correlate the degradative phenomena evolution, a comparison of LFV1 and LFV2 versus water uptake trends is schemed in Figure 11. Concerning Stage 1, LFV2 was equal to about 1.4, thus indicating peak variation of about 40%, twice the variation of the first peak. In addition, the slope of the LFV2 trend was noticeably higher than that evidenced for LFV1. These experimental results further confirmed that the main limiting factor in the application of NFRPs in humid or wet conditions is represented by the worsening effect on the fiber–matrix adhesion of these environments. This degradation phenomenon is much more sensitive than other ones such as matrix softening, which, although giving an appreciable contribution on aging degradation, can be considered, in this case, a sub-degradation step of the global degradation mechanisms. Further studies will be aimed to better discriminate among the different damage evolution stages occurring in critical environmental conditions, such as salt-fog spray. This will provide added value in the identification of synergistic actions between various degradative mechanisms that contribute to the degradation of the laminate. However, the achieved results are promising and potentially suitable to integrate the design knowledge of natural composite laminates and their durability performance under specific environmental conditions during the design phase. Polymers 2020, 12, x FOR PEER REVIEW 14 of 18
first stage, i.e., LFV2 values equal to about 1.4 for water uptake lower than 12%. These experimental results confirmed that the fiber–matrix adhesion weakening was strongly correlated with the water absorption experienced by the laminate in the beginning phase of the salt‐fog exposition. Therefore, this stage can be considered a discriminating factor concerning the micro‐mechanical transition phenomena of the immobilized polymer layer between fiber and matrix (i.e., fiber–matrix interphase). On this basis, it is reasonable to consider that the water diffusion inside the composite structure mainly depended on a weak fiber–matrix adhesion that, in turn, represented the main factor activating degradative phenomena. The transition between the two degradative stages was identified, also in this case, at water uptake values equal to 12%. Beyond this threshold limit, the LFV2 index became higher than 2 (i.e., up to 3.5 for Flax‐AR samples, for instance), thus indicating a serious loss of the adhesion between the main components of the composite (i.e., fiber and matrix). Due to the above reasons, composites having the weakest fiber–matrix adhesion, such as Flax‐ AR and Jute‐T, evidenced the highest increments in loss factor peaks as well as the greatest moisture absorption during their exposition to the salt‐fog environment. On the contrary, the beneficial effect of the NaHCO3 treatment on the adhesion between flax fibers and epoxy resin allowed composites such as Flax‐T and F‐Hybrid T to reduce their moisture absorption and, as a consequence, to better retain their damping peaks. Similarly, composites with inherently good fiber–matrix adhesion, such as Jute‐AR and J Hybrid‐AR, showed low moisture adsorption and slight variations in both loss factor peaks, also after long exposition time in the salt‐fog environment. With the aim to better correlate the degradative phenomena evolution, a comparison of LFV1 and LFV2 versus water uptake trends is schemed in Figure 11. Concerning Stage 1, LFV2 was equal to about 1.4, thus indicating peak variation of about 40%, twice the variation of the first peak. In addition, the slope of the LFV2 trend was noticeably higher than that evidenced for LFV1. These experimental results further confirmed that the main limiting factor in the application of NFRPs in humid or wet conditions is represented by the worsening effect on the fiber–matrix adhesion of these
Polymersenvironments.2020, 12, 716 This degradation phenomenon is much more sensitive than other ones such as matrix15 of 18 softening, which, although giving an appreciable contribution on aging degradation, can be considered, in this case, a sub‐deradation step of the global degradation mechanisms.
FigureFigure 11. 11. SchemeScheme ofof LFVLFV11 and LFV22 index versus water uptake trends comparison.
4. ConclusionsFurther studies will be aimed to better discriminate among the different damage evolution stagesSinceoccurring humid in environments critical environmental can be considered conditions, as thesuch most as salt critical‐fog factorsspray. This reducing will provide the service added life ofvalue natural in fiber-reinforcedthe identification composites, of synergistic the present actions paperbetween aims to various deeper understanddegradative howmechanisms the exposition that tocontribute aggressive to environmentsthe degradation such of as the salt-fog laminate. spray However, conditions the can achieved influence results the dynamic are promising mechanical and responsepotentially of thesesuitable materials. to integrate the design knowledge of natural composite laminates and their durabilityWithin performance this scope, the under dampingspecific properties environmental of flax, jute conditions and hybrid during flax/ jutethe design epoxy basedphase. composites were analyzed as a function of their exposition time to salt-fog. Furthermore, it was investigated how sodium bicarbonate treatment can allow these composites to better retain their dynamic mechanical response under aging conditions. By way of summarizing, it can be drawn from the experimental campaign that:
All the NFRP materials (i.e., regardless the stacking sequence) experienced decreases of the − storage modulus and glass transition temperature, as well as the increase of the loss factor peaks with salt-fog exposition time and moisture content; Full laminates reinforced with flax fibers or hybrid laminates having more flax fabrics than jute − ones in their stacking sequence evidenced a clear effectiveness of NaHCO3 treatment on the retention of their damping properties during exposition to salt-fog environments; Conversely, the proposed treatment had no beneficial effect on jute-based laminates (in terms − of preservation of damping properties), thus evidencing that the efficacy of treatment is strictly related to the fiber’s chemical composition; A relationship between the decrease of the damping properties of the composites and their − progressive increase of water absorption during the salt-fog exposition was evaluated in order to better discriminate how mechanical parameters are affected by degradative phenomena, thus indicating that the fiber–matrix interfacial water diffusion plays a relevant role in the degradation triggering and propagation.
Author Contributions: Data curation, V.F., C.S. and L.C.; formal analysis, V.F. and L.C.; investigation, C.S.; methodology, V.F. and L.C.; validation, V.F. and L.C.; writing—original draft, V.F., writing—review and editing, V.F. and L.C.; visualization, V.F. and L.C. and; supervision, L.C. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest. Polymers 2020, 12, 716 16 of 18
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