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Effect of moisture on elastic and viscoelastic properties of epoxy and epoxy-based carbon fibre reinforced plastic filled with multiwall carbon nanotubes
Composites: Part A 90 (2016) 522–527
Contents lists available at ScienceDirect
Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Effect of moisture on elastic and viscoelastic properties of epoxy and epoxy-based carbon fibre reinforced plastic filled with multiwall carbon nanotubes Tatiana Glaskova-Kuzmina a,⇑, Andrey Aniskevich a, Alfonso Martone b, Michele Giordano b, Mauro Zarrelli b a b
Institute for Mechanics of Materials, University of Latvia, Aizkraukles 23, Riga LV-1006, Latvia Institute for Composite and Biomedical Materials, National Research Council, Portici, NA, Italy
a r t i c l e
i n f o
Article history: Received 3 March 2016 Received in revised form 18 August 2016 Accepted 20 August 2016 Available online 22 August 2016 Keywords: A. Polymer-matrix composites (PMCs) A. Carbon fibres B. Creep D. Moisture
a b s t r a c t In this study, we investigated the peculiarities of moisture absorption and moisture-induced effects on the elastic and viscoelastic flexural properties of epoxy resin and carbon fibre reinforced plastic (CFRP) filled with multiwall carbon nanotubes (MWCNTs). Short-term cyclic creep-recovery tests of moistened epoxy and CFRP filled with MWCNTs revealed improvements in creep resistance for both materials. The addition of MWCNTs to the epoxy resin suppressed the moisture absorption by the material, causing a reduction in the diffusion coefficient by 31% and equilibrium moisture content by 15%. The addition of MWCNTs reduced the flexural strength of moistened epoxy and CFRP samples by approximately half, and also lowered the flexural modulus by 1.4 and 3 times, elastic strain by 1.25 and 1.04 times, viscoelastic strain by 1.39 and 1.03 times, and plastic strain by 2.68 and 1.60 times, respectively. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, the use of polymer composites and fibre-reinforced plastics (FRPs) has increased drastically, owing to their application in fabricating various small and large components of aircrafts, helicopters, spacecrafts, boats, ships, offshore platforms, automobiles, chemical processing equipment, sports goods, and civil infrastructures such as buildings and bridges [1]. The investigation of effect of moisture on the mechanical properties of polymers and polymer-based FRPs is crucial as polymer composites are usually exposed to the influence of external factors such as constant or time-varying humidity. These external factors can cause the deteriorations of the functional, physical, and mechanical properties of polymer-based composite materials, due to physical and chemical transformations [2,3]. The property sensitivity to such degradation greatly depends on the environment and unique response of each structural component [3]. Therefore, besides the mechanical loads acting on the polymerbased components, high moisture levels and extreme and sudden changes in temperature should be considered at the analysis and design stages. According to the results published by ESPEC [4],
⇑ Corresponding author. E-mail address: [email protected] (T. Glaskova-Kuzmina). http://dx.doi.org/10.1016/j.compositesa.2016.08.026 1359-835X/Ó 2016 Elsevier Ltd. All rights reserved.
about 60% of all environmentally induced failures occur because of incidental temperature and humidity changes. The addition of multiwall carbon nanotubes (MWCNTs) to polymers and polymer-based FRPs offers the possibility to reduce the negative effect of moisture absorption. However, their long-term deformability and strength under the action of loads at elevated and varying moisture levels must be evaluated. It was experimentally confirmed that, upon addition of only 0.3 wt% MWCNTs, the water absorption of epoxy resin decreased by almost three times, compared with that of the neat polymer, and consequently its resistance to hydrothermal ageing improved considerably [5]. This effect was explained by considering the hindering action of the nanofiller on the intermolecular movements of the epoxy [5]. Besides the main FRP constituents, i.e., the polymer matrix and fibres, the interface/interphase plays a very crucial role on the performance and reliability of FRPs. Good interfacial properties are essential to ensure the efficient load transfer from the matrix to the reinforcement, helping reduce the stress concentration and improving the overall sustainability of the FRP mechanical properties [3]. The reduced glass transition temperature of the interphase may induce low modulus area. Additionally, it may lead to high fracture toughness at ambient temperatures or humidities, but significantly reduced performances at high temperatures or humidities [6,7].
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A very limited number of papers concerning the experimental investigation of environmental effects on the time-dependent mechanical properties of MWCNT-filled FRPs have been published. Nevertheless, the urgency of such research is caused by the need to predict the service life and durability of the materials under the action of environmental factors. Therefore, the current research focused on the creep characteristics of moistened epoxy and epoxy-based CFRP filled with MWCNTs. This study aimed to determine the peculiarities of moisture absorption and moistureinduced effects on the elastic and viscoelastic flexural properties of these nanomodified materials. As shown in a previous work, MWCNTs dispersed in the epoxy matrix improved the creep resistance of CFRP [8]; however, the moisture-induced effects on the elastic and viscoelastic properties of the CFRP/MWCNT were not evaluated.
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all tested materials. Five specimens were used for each material type, and the averaged values are presented here. All specimens reached the equilibrium moisture content (saturation) after 7 months. Quasi-static and cyclic three-point bending creep tests were performed on fully saturated specimens. The test procedure for dry materials has been previously described in [8]. Four cycles of gradually increasing stress, equalled to 25%, 50%, 75%, and 90% of the flexural strength, were used for loading during a period of 30 min, followed by 30 min of unloading. Notably, the forces applied to the moistened materials were lower, as the moisture caused an inherent reduction in the flexural strength for all tested materials. Thus, the effects of absorbed moisture on the elastic and viscoelastic properties of EP, NC, CFRP, and CFRP-NC were determined.
3. Results and discussion 2. Material and methods 3.1. Moisture absorption of EP, NC, CFRP, and CFRP-NC In this work, neat monocomponent RTM6 epoxy resin (provided by Hexcel Composites) and laminated CFRP ([0/45/90/45]2) were investigated. The epoxy resin was filled with 1 wt% MWCNT (length: 5–9 lm, diameter: 110–170 nm, aspect ratio: 30–80; provided by Sigma–Aldrich). Such nanomodified epoxy was further used for the impregnation of CFRP by a vacuum infusion process. The processing conditions were exactly the same as those reported in [8]. Furthermore, these materials will be denoted as EP (epoxy resin), NC (nanocomposite; epoxy resin filled with 1 wt% MWCNTs), CFRP, and CFRP-NC (CFRP impregnated with NC). The dispersion of MWCNTs in the epoxy matrix was rather homogeneous, as shown at different magnifications (200 and 500) in [8]. However, agglomerates of MWCNTs in the size range of 25–100 lm were found; they exhibited different shapes, but possessed aspect ratios roughly close to unity. Certainly, their formation could negatively influence the infusion process of the fibrous reinforcement. Additionally, their presence might diminish any benefits to the improvement of mechanical properties of multiscale composite material associated with the nanoscale dimensions. The dimensions of all test specimens were 150 ± 2, 10 ± 1, and 2 ± 0.2 mm to ensure one-dimensional (1D) mode of diffusion. Owing to the layup in the CFRP, the test specimens were regarded as transversely isotropic materials. In this case, one plane is isotropic, while the direction normal to that plane (thickness of the specimens) defines the axis with different properties. Thus, the diffusivity tensor includes two independent components, i.e., longitudinal and transverse diffusivities [9]. As CFRP and CFRP-NC specimens had the same dimensions and cutting direction, the diffusivity of the specimens could be considered as effective diffusivity, which included both components. Before starting the moisture absorption tests, all specimens were conditioned in a desiccator with silica gel at the relative humidity of 24% to remove the absorbed moisture until all specimens showed no change in mass. Subsequently, they were placed in a desiccator at the relative humidity of 98%, created by using a saturated solution of K2SO4. Moisture desorption and sorption experiments were performed at room temperature (T = 20 °C) to prevent temperature-induced effects on the absorption process. For the investigation of the moisture absorption kinetics, the specimens were periodically removed from the desiccator, air dried for 5 min, and then weighed by using a Mettler Toledo XS205DU balance (accuracy: 0.05 mg). A period of 5 min was sufficient to remove the surface moisture, and no significant desorption from the bulk of specimens was observed. The results obtained in the moisture absorption experiments were accurately compared for
Notably, two states of water can be observed in polymer-based systems upon moisture absorption: (1) unbound free water, which fills the nanovoids (also called free volume), not inducing swelling; (2) hydrogen-bounded water, which causes swelling of the polymers [3,10]. Composites with a high content of free volume have considerable space available for diffusing water molecules, resulting in high moisture uptake; this, in turn, may enhance the negative effect of moisture on the mechanical properties of composite systems [5]. In FRP systems, moisture is introduced via diffusion flow along the fibre/matrix interface, or conveyed via microcracks and voids, leading to the diffusion in the surrounding matrix [3]. Thus, when MWCNTs are well dispersed within the epoxy, the reduction in the free volume may cause significant improvements in the sorption and mechanical characteristics of the composite system. The moisture absorption curves obtained for EP, NC, CFRP, and CFRP-NC are given in Fig. 1. The moisture content w(t) was defined as the mass difference between moistened mt (time-varying) and initial m0 specimens, normalized to the initial mass of specimens according to the relation:
wðtÞ ¼
mt m0 100%: m0
ð1Þ
For the description of moisture absorption curves of epoxies and epoxy-based composites (including CFRP), classical second Fick’s law is usually applied [10–12]. In the case of the 1D diffusion
Fig. 1. Moisture uptake by epoxy resin (s), nanocomposite (NC) (d), carbon fibre reinforced plastic (CFRP) (e), and CFRP impregnated with NC (r). Lines are calculated by using Eq. (2).
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mode (when the thickness of the test specimens is significantly smaller than the length and width), the moisture content is expressed as follows:
wðtÞ ¼ w1
1 2ðw1 w0 Þ X ð1 ð1Þk Þ
p2
k¼1
2 ! pk exp Dt : h
k
2
2
ð2Þ
Here, w0 and w1 are the initial and equilibrium moisture contents in the specimens, respectively, while h and D are the thickness and diffusion coefficient of the specimens, respectively. As seen from Fig. 1, Fick’s model was in good agreement with the experimental data for all tested materials (i.e., EP, NC, CFRP, and CFRP-NC). For CFRP and CFRP-NC, the sorption process was slower, and the level of moisture uptake was lower than that of the EP and NC systems. This was due to the presence of a significant content (60 vol%) of moisture-impenetrable carbon fibres and, consequently, to the small amount of free volume in FRP available for water molecules diffusing through the test specimens. The data scattering from the averaged values of moisture content was moderate, and a standard deviation of less than 10% was observed. EP specimens had the most scattered data, possibly due to the manual mixing process and the consequent appearance of different inhomogeneities in the material (areas of different density, pores, etc.). In turn, NC samples showed a minor standard deviation from the averaged value of the moisture content. The low values of standard deviation revealed the homogeneous dispersion of MWCNTs in the epoxy as well as their compatibility with EP [13]. Classical Fick’s model is easy to use as it contains only two parameters: the diffusivity D and equilibrium moisture content w1. When the sorption curve is given on a diagram with abscissa p axis t (see Fig. 1), the initial section of this diagram is a line passing through the origin of the coordinates. Thus, by using experimental data, the diffusivity can be determined from its inclination:
D¼
ph2 wðtÞ w0 16t
w1 w0
2 :
ð3Þ
Generally, the equilibrium moisture content w1 is experimentally found as the maximal moisture content observed in the specimen. For all tested materials, the parameters of the sorption process, i.e., the diffusivity found according to Eq. (3) and equilibrium moisture content, are shown in Fig. 2. As shown in Fig. 2, the sorption process observed for the NC was slower than that seen for the EP. The NC diffusivity decreased almost by one third (31%), compared with that of the EP. In addition, the equilibrium moisture content of the materials investigated decreased by 15% in the case of NC, whereas it remained
Fig. 2. Diffusivity D (left ordinate axis) and equilibrium moisture content w1 (right ordinate axis) of epoxy resin, nanocomposite (NC), carbon fibre reinforced plastic (CFRP), and CFRP impregnated with NC.
almost identical (reduced by 0.47%) for CFRP-NC. This effect could be associated with the aspect ratio of the MWCNTs, i.e., 30–80, which led not only to an increased tortuosity of the path of the water molecules upon moisture diffusion into the diffused system, but also to a restriction of the molecular dynamics for the polymer chains surrounding the nanoparticles, causing a retardation of their relaxation. The improved sorption characteristics also indicated the good adhesion between EP and MWCNTs in NC, and MWCNTs and carbon fibres in CFRP-NC systems [5]. For the CFRP-NC, the retarded relaxation of the epoxy chains was less dominant, owing to the rigid network of carbon fibres. Therefore, the effect of the MWCNTs on the sorption characteristics was less prominent in CFRP-NC. Notably, such a reduction in the diffusivity was also characteristic to other kinds of nanoparticles having high aspect ratio. Similar results were obtained previously by the authors for epoxy filled with different contents of montmorillonite clay [14]. Clearly, besides the different chemical composition, montmorillonite clay has a layered structure, and the most pronounced reduction in the moisture diffusion process was obtained in the case of fully exfoliated clay platelets. Nevertheless, the analysis of the MWCNT dispersion in the epoxy [8] confirmed that the MWCNTs formed microsized agglomerates, likely minimizing the prominent effects of the sorption properties of the NC and CFRP-NC systems. The improved barrier characteristics for polymers filled with nanoparticles still offer a possibility to achieve improvements also in the long-term deformability of such materials. To this end, further investigations and discussions should be conducted. 3.2. Effect of moisture on the elastic flexural properties of EP, NC, CFRP, and CFRP-NC The representative stress-strain curves for three-point bending tests of four different materials (EP, NC, CFRP, and CFRP-NC) in the ‘‘dry state” were fully discussed in [8]. In this section, the attention is focused on the effects induced by the absorbed moisture on the elastic flexural properties of these materials. Flexural strength was defined as the maximal value of stress achieved by the specimens. The flexural modulus was calculated from the slope of a secant line between 0.05% and 0.25% strains on the stress–strain plot. The representative stress–strain curves for EP, NC, CFRP, and CFRP-NC in dry and moistened states are given in Fig. 3a. For all materials, the relative changes in flexural strength and modulus, due to absorbed moisture, are shown in Fig. 3b. In contrast to dry materials, for which only insignificant ‘‘nanoeffects” on the elastic flexural properties were observed [8], moistened NC and CFRP-NC revealed improved resistance to moisture-induced reduction in both flexural characteristics. As seen in Fig. 3b, the flexural strengths for moistened EP, NC, CFRP, and CFRP-NC decreased by 16%, 8%, 12%, and 6%, respectively. Slightly higher variations of 29%, 21%, 18%, and 6%, were recorded for the flexural moduli, respectively. Thus, the addition of MWCNTs to EP and CFRP led to an overall reduction in the detrimental effect of moisture absorption on these characteristics. The decrease in the flexural strength of NC and CFRP-NC was half that of the EP and CFRP systems, whereas the flexural modulus was 1.4 and 3 times lower than that of the EP and CFRP systems. As for the sorption characteristics, such results can be attributed to the reduction in the free volume in the composite system by the addition of MWCNTs. This resulted in the reduced moisture absorption and subsequent mitigation of the negative effect of the absorbed moisture. The improved mechanical properties of the moistened NC and CFRP-NC systems, due to the addition of MWCNTs, also confirmed that the adhesion between epoxy resin, MWCNTs, and carbon fibres was sufficiently good, ensuring better stress transfer than that of EP and CFRP. Nevertheless, the environ-
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Fig. 3. Representative stress-strain curves of epoxy resin (solid lines to the right), nanocomposite (NC; dashed lines to the right), carbon fibre reinforced plastic (CFRP; solid lines to the left), and CFRP impregnated with NC (dashed lines to the left) in dry (1) and moistened (2) states (a), and the effect of absorbed moisture on relative change of flexural strength and modulus (b).
mental exposure resulted in reduced interfacial stress transmissibility due to matrix plasticization, chemical changes, and mechanical degradation in the interphase/interface area [3]. Notably, the results obtained for the tensile tests of the epoxy/ MWCNT composites after hydrothermal ageing [15] showed that the plasticization ability of the absorbed moisture resulted in almost the same reduction in the elastic modulus and strength (5–8% and 18–22%, respectively). This was attributed to the altered internal stress conditions and debonding due to the volume expansion among nanoparticles and polymer matrix [15,16]. However, although the effect of moisture on the elastic flexural properties of EP and CFRP was relevant, it could be mitigated by the addition of MWCNTs. These results could be further exploited for applications that involve long-term use and require stable mechanical properties in natural environments with relevant concentration of humidity or aggressive liquid, e.g., transportation, construction, etc. 3.3. Effect of moisture on the viscoelastic flexural properties of EP, NC, CFRP, and CFRP-NC Moisture, similarly to stress and temperature, leads to the acceleration of creep processes in materials. Flexural creep experiments were performed in four cycles with step-by-step increasing stress. The effect of the nanofiller was expected to be more noticeable at the highest values of stress and moisture content. The effect of cycling was investigated as in [8] by comparing the main strain components (elastic, viscoelastic, and plastic) for all tested materials in dry and moistened states.
Fig. 4. Representative creep curves for epoxy resin (solid lines) and nanocomposite (dashed lines) in dry (1) and moistened (2) states at the highest stress level (0.75% of flexural strength).
The creep curves for EP and NC samples in dry and moistened states at the highest stress levels are shown in Fig. 4. Obviously, absorbed moisture essentially acted as plasticizer for the system, leading to an increase in deformability. The addition of MWCNTs to the EP system reduced such a negative effect associated with the moisture absorption. In fact, the NC and CFRP-NC samples had lower values for all creep strain components than the EP and CFRP samples (see Fig. 5). Thus, the nanomodified materials exhibited improved creep resistance in the moistened state. The addition of MWCNTs to EP and CFRP resulted in almost equivalent improvements in creep resistance in relation to the stress applied: the elastic strain was reduced by 1.25 and 1.04 times, viscoelastic strain by 1.39 and 1.03 times, and plastic strain by 2.68 and 1.60 times, respectively. The reduction in viscoelastic and plastic creep strains for both NC and CFRP-NC specimens could be attributed to the removal of MWCNTs from the agglomerates and subsequent destruction of the agglomerates. This occurred upon cyclic and increasing loadings, leading to improved matrix–filler interfacial interactions [8]. The creep strain analysis, conducted for all materials in dry (e0) and moistened states (ew), allowed investigating the relative moisture-induced effect (ew/e0) at the highest value of applied stress (Fig. 6). For EP and NC, the effect of moisture, normalized to the creep strain of the dry state, resulted in the increase at the mean: by 1.33 and 1.32 times for elastic strain, 1.48 and 1.24 times for viscoelastic strain, and 1.53 and 1.28 times for plastic strain, respectively. For CFRP and CFRP-NC, the effect was quite insignificant for elastic strain—increase at the mean of 1.09 and 1.03 times, respectively—and more pronounced for viscoelastic—increase at the mean by 1.52 and 1.33 times, respectively—and plastic—increase at the mean by 1.45 and 1.39 times, respectively—strains. Nevertheless, the values of viscoelastic and plastic strains for CFRP were so small (<0.007%) that such effect can be neglected. Notably, the addition of MWCNTs resulted in a reduction in the effect of moisture on creep strains, due to the retardation of the relaxation processes upon creep in all considered cases. Thus, the addition of MWCNTs promoted the formation of restriction sites for relatively slow regrouping of polymer macromolecules upon creep. This result provides the possibility to improve the creep resistance for both epoxy and CFRP not only in the dry state, but also in the moistened state. The time-stress superposition principle and the Boltzmann– Volterra linear integral equation can be used for the description of a series of creep curves at various stresses for EP and NC in the moistened state. The master curves of creep compliance for the materials in the moistened state were compared with those obtained for the materials in the dry state (Fig. 7).
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Fig. 5. Effect of moisture on elastic ee (a), viscoelastic eve (b), and plastic ep (c) strains of moistened epoxy resin (e) and nanocomposite (r) (NC; left ordinate axis), and carbon fibre reinforced plastic (CFRP) (h) and CFRP impregnated with NC (j) (right ordinate axis).
Fig. 6. Creep strains for epoxy resin, nanocomposite (NC), carbon fibre reinforced plastic (CFRP), and CFRP impregnated with NC in moistened state as normalized to dry state (ee, elastic strain; eve, viscoelastic strain; ep, plastic strain).
Obviously, the master curves for moistened EP and NC were located above the master curves for the same materials in the dry state. Nevertheless, the master curve for moistened NC was positioned under the master curve of moistened EP. This effect increased with time and provided an improvement in the longterm creep resistance of MWCNT-filled epoxy. Consequently, the operation time of such materials will be longer under both dry and humid conditions. The results obtained for the time-stress shift factor (Fig. 8) for moistened epoxy and NC completed the above analysis of the improved creep resistance due to the addition of MWCNTs. It should be noted that the standard deviation in Fig. 8 for the time-stress shift factor of moistened NC at the highest stress levels was only 0.05, and was located inside the markers. Evidently, the time-stress shift factor for the nanomodified epoxy specimens,
Fig. 7. Master curves for epoxy resin and nanocomposite in dry (s and D) and moistened (d and ▲) states, obtained using the time-stress superposition principle.
having absorbed moisture, was reduced at all applied stresses. These results provide the possibility to expand the use of such materials to different applications that involve long-term use and require stable mechanical properties under humid conditions.
4. Conclusions Short-term cyclic creep-recovery tests of moistened EP, NC, CFRP, and CFRP-NC showed the reduction in all creep strains in nanomodified materials (NC and CFRP-NC). The improvement in creep resistance could be explained by considering the decreased mobility of polymer chains both in dry and moistened states, as the MWCNTs acted as restriction sites. On the one hand, the addition of moisture-impenetrable MWCNTs to the epoxy resin caused the retardation of moisture dif-
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Acknowledgments The research of A. Aniskevich was supported by the Latvian state research programme under grant agreement ‘‘IMATEH”. References
Fig. 8. Time-stress shift factor vs difference in applied stress, starting from the stress of the first cycle for epoxy resin and nanocomposite in dry (-s- and -h-) and moistened (-d- and -j-) states, respectively.
fusion, resulting in the reduction in the diffusion coefficient by 31% and equilibrium moisture content by 15%. Minor effect on the same sorption characteristics was found for epoxy-based CFRP. On the other hand, MWCNTs, having high stiffness and strength, improved the flexural properties of these materials: the flexural strengths of NC and CFRP-NC were reduced twice, the flexural moduli by 1.4 and 3 times, respectively, while the creep strains were reduced by 1.25 and 1.04 times (elastic), 1.39 and 1.03 times (viscoelastic), and 2.68 and 1.60 times (plastic), respectively. The improved mechanical properties of the moistened NC and CFRP-NC showed that the adhesion between the epoxy resin, MWCNTs, and carbon fibres was sufficiently good, and ensured a superior stress transfer, compared with that of EP and CFRP. Clearly, the moisture-induced effects that led to such significant reduction in the mechanical characteristics were crucial. This was ascribed to the reduced interfacial stress transmissibility due to matrix plasticization, chemical changes, and mechanical degradation in the interphase/interface area. Further improvements in barrier characteristics and stability against environmental factors can be obtained by optimization of the dispersion process and chemical functionalization of MWCNTs. In this way, the agglomeration of nanoparticles can be reduced, and the interfacial strength between them and the polymer matrix can be improved. Thus, owing to the addition of stiff and moisture-impenetrable MWCNTs, the negative effect of absorbed moisture was reduced. As a result, MWCNT-filled CFRPs can be used as structural components in different applications requiring high performance under humid conditions.
[1] Hollaway LC. A review of the present and future utilisation of FRP composites in the civil infrastructure with reference to their important in-service properties. Constr Build Mater 2010;24:2419–45. [2] Aniskevich KK, Glaskova TI, Aniskevich AN, Faitelson YA. Effect of moisture on the viscoelastic properties of an epoxy-clay nanocomposite. Mech Compos Mater 2010;46:573–82. [3] Sethi S, Ray BC. Environmental effects on fibre reinforced polymeric composites: evolving reasons and remarks on interfacial strength and stability. Adv Colloid Interface 2015;217:43–67. [4] ESPEC Technology Report Nr. 1. Special issue: Evaluating Reliability. [5] Starkova O, Chandrasekaran S, Prado Lasa, Tölle F, Mülhaupt R, Schulte K. Hydrothermally resistant thermally reduced graphene oxide and multi-wall carbon nanotube based epoxy nanocomposites. Polym Degrad Stabil 2013;98:519–26. [6] Ray BC, Rathore D. Durability and integrity studies of environmentally conditioned interfaces in fibrous polymeric composites: critical concepts and comments. Adv Colloid Interface 2014;209:68–83. [7] Theocaris P, Papanicolaou G. Interrelation between moisture absorption, mechanical behavior, and extent of boundary interface in particulate composites. J Appl Polym Sci 1983;28:3145–53. [8] Glaskova-Kuzmina T, Aniskevich A, Zarrelli M, Martone A, Giordano M. Effect of filler on the creep characteristics of epoxy and epoxy-based CFRPs containing multi-walled carbon nanotubes. Compos Sci Technol 2014;100:198–203. [9] Bank LC. Properties of FRP reinforcements for concrete. In: Nanni A, editor. Fiber-reinforced-plastic (FRP) reinforcement for concrete structures. Properties and applications. Amsterdam: Elsevier Science Publishers BV; 1993. p. 59–88. [10] Glaskova TI, Guedes RM, Morais JJ, Aniskevich AN. A comparative analysis of moisture transport models as applied to an epoxy binder. Mech Compos Mater 2007;43(4):377–88. [11] Grammatikos SA, Zafari B, Evernden MC, Mottram JT, Mitchels JM. Moisture uptake characteristics of a pultruded fibre reinforced polymer flat sheet subjected to hot/wet aging. Polym Degrad Stab 2015;121:407–19. [12] Jiang X, Kolstein H, Bijlaard FSK. Moisture diffusion in glass–fiber-reinforced polymer composite bridge under hot/wet environment. Compos: Part B 2013;45:407–16. [13] Starkova O, Buschorn ST, Mannov E, Schulte K, Aniskevich A. Water transport in epoxy/MWCNT composites. Eur Polym J 2013;49:2138–48. [14] Glaskova T, Aniskevich A. Moisture absorption by epoxy/montmorillonite nanocomposite. Compos Sci Technol 2009;69:2711–5. [15] Starkova O, Mannov E, Schulte K, Aniskevich A. Strain-dependent electrical resistance of epoxy/MWCNT composite after hydrothermal aging. Compos Sci Technol 2015;117:107–13. [16] Lee JH, Rhee KY, Lee JH. Effects of moisture absorption and surface modification using 3-aminopropyltriethoxysilane on the tensile and fracture characteristics of MWCNT/epoxy nanocomposites. Appl Surf Sci 2010;256:7658–67.
Contents lists available at ScienceDirect
Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Effect of moisture on elastic and viscoelastic properties of epoxy and epoxy-based carbon fibre reinforced plastic filled with multiwall carbon nanotubes Tatiana Glaskova-Kuzmina a,⇑, Andrey Aniskevich a, Alfonso Martone b, Michele Giordano b, Mauro Zarrelli b a b
Institute for Mechanics of Materials, University of Latvia, Aizkraukles 23, Riga LV-1006, Latvia Institute for Composite and Biomedical Materials, National Research Council, Portici, NA, Italy
a r t i c l e
i n f o
Article history: Received 3 March 2016 Received in revised form 18 August 2016 Accepted 20 August 2016 Available online 22 August 2016 Keywords: A. Polymer-matrix composites (PMCs) A. Carbon fibres B. Creep D. Moisture
a b s t r a c t In this study, we investigated the peculiarities of moisture absorption and moisture-induced effects on the elastic and viscoelastic flexural properties of epoxy resin and carbon fibre reinforced plastic (CFRP) filled with multiwall carbon nanotubes (MWCNTs). Short-term cyclic creep-recovery tests of moistened epoxy and CFRP filled with MWCNTs revealed improvements in creep resistance for both materials. The addition of MWCNTs to the epoxy resin suppressed the moisture absorption by the material, causing a reduction in the diffusion coefficient by 31% and equilibrium moisture content by 15%. The addition of MWCNTs reduced the flexural strength of moistened epoxy and CFRP samples by approximately half, and also lowered the flexural modulus by 1.4 and 3 times, elastic strain by 1.25 and 1.04 times, viscoelastic strain by 1.39 and 1.03 times, and plastic strain by 2.68 and 1.60 times, respectively. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, the use of polymer composites and fibre-reinforced plastics (FRPs) has increased drastically, owing to their application in fabricating various small and large components of aircrafts, helicopters, spacecrafts, boats, ships, offshore platforms, automobiles, chemical processing equipment, sports goods, and civil infrastructures such as buildings and bridges [1]. The investigation of effect of moisture on the mechanical properties of polymers and polymer-based FRPs is crucial as polymer composites are usually exposed to the influence of external factors such as constant or time-varying humidity. These external factors can cause the deteriorations of the functional, physical, and mechanical properties of polymer-based composite materials, due to physical and chemical transformations [2,3]. The property sensitivity to such degradation greatly depends on the environment and unique response of each structural component [3]. Therefore, besides the mechanical loads acting on the polymerbased components, high moisture levels and extreme and sudden changes in temperature should be considered at the analysis and design stages. According to the results published by ESPEC [4],
⇑ Corresponding author. E-mail address: [email protected] (T. Glaskova-Kuzmina). http://dx.doi.org/10.1016/j.compositesa.2016.08.026 1359-835X/Ó 2016 Elsevier Ltd. All rights reserved.
about 60% of all environmentally induced failures occur because of incidental temperature and humidity changes. The addition of multiwall carbon nanotubes (MWCNTs) to polymers and polymer-based FRPs offers the possibility to reduce the negative effect of moisture absorption. However, their long-term deformability and strength under the action of loads at elevated and varying moisture levels must be evaluated. It was experimentally confirmed that, upon addition of only 0.3 wt% MWCNTs, the water absorption of epoxy resin decreased by almost three times, compared with that of the neat polymer, and consequently its resistance to hydrothermal ageing improved considerably [5]. This effect was explained by considering the hindering action of the nanofiller on the intermolecular movements of the epoxy [5]. Besides the main FRP constituents, i.e., the polymer matrix and fibres, the interface/interphase plays a very crucial role on the performance and reliability of FRPs. Good interfacial properties are essential to ensure the efficient load transfer from the matrix to the reinforcement, helping reduce the stress concentration and improving the overall sustainability of the FRP mechanical properties [3]. The reduced glass transition temperature of the interphase may induce low modulus area. Additionally, it may lead to high fracture toughness at ambient temperatures or humidities, but significantly reduced performances at high temperatures or humidities [6,7].
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A very limited number of papers concerning the experimental investigation of environmental effects on the time-dependent mechanical properties of MWCNT-filled FRPs have been published. Nevertheless, the urgency of such research is caused by the need to predict the service life and durability of the materials under the action of environmental factors. Therefore, the current research focused on the creep characteristics of moistened epoxy and epoxy-based CFRP filled with MWCNTs. This study aimed to determine the peculiarities of moisture absorption and moistureinduced effects on the elastic and viscoelastic flexural properties of these nanomodified materials. As shown in a previous work, MWCNTs dispersed in the epoxy matrix improved the creep resistance of CFRP [8]; however, the moisture-induced effects on the elastic and viscoelastic properties of the CFRP/MWCNT were not evaluated.
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all tested materials. Five specimens were used for each material type, and the averaged values are presented here. All specimens reached the equilibrium moisture content (saturation) after 7 months. Quasi-static and cyclic three-point bending creep tests were performed on fully saturated specimens. The test procedure for dry materials has been previously described in [8]. Four cycles of gradually increasing stress, equalled to 25%, 50%, 75%, and 90% of the flexural strength, were used for loading during a period of 30 min, followed by 30 min of unloading. Notably, the forces applied to the moistened materials were lower, as the moisture caused an inherent reduction in the flexural strength for all tested materials. Thus, the effects of absorbed moisture on the elastic and viscoelastic properties of EP, NC, CFRP, and CFRP-NC were determined.
3. Results and discussion 2. Material and methods 3.1. Moisture absorption of EP, NC, CFRP, and CFRP-NC In this work, neat monocomponent RTM6 epoxy resin (provided by Hexcel Composites) and laminated CFRP ([0/45/90/45]2) were investigated. The epoxy resin was filled with 1 wt% MWCNT (length: 5–9 lm, diameter: 110–170 nm, aspect ratio: 30–80; provided by Sigma–Aldrich). Such nanomodified epoxy was further used for the impregnation of CFRP by a vacuum infusion process. The processing conditions were exactly the same as those reported in [8]. Furthermore, these materials will be denoted as EP (epoxy resin), NC (nanocomposite; epoxy resin filled with 1 wt% MWCNTs), CFRP, and CFRP-NC (CFRP impregnated with NC). The dispersion of MWCNTs in the epoxy matrix was rather homogeneous, as shown at different magnifications (200 and 500) in [8]. However, agglomerates of MWCNTs in the size range of 25–100 lm were found; they exhibited different shapes, but possessed aspect ratios roughly close to unity. Certainly, their formation could negatively influence the infusion process of the fibrous reinforcement. Additionally, their presence might diminish any benefits to the improvement of mechanical properties of multiscale composite material associated with the nanoscale dimensions. The dimensions of all test specimens were 150 ± 2, 10 ± 1, and 2 ± 0.2 mm to ensure one-dimensional (1D) mode of diffusion. Owing to the layup in the CFRP, the test specimens were regarded as transversely isotropic materials. In this case, one plane is isotropic, while the direction normal to that plane (thickness of the specimens) defines the axis with different properties. Thus, the diffusivity tensor includes two independent components, i.e., longitudinal and transverse diffusivities [9]. As CFRP and CFRP-NC specimens had the same dimensions and cutting direction, the diffusivity of the specimens could be considered as effective diffusivity, which included both components. Before starting the moisture absorption tests, all specimens were conditioned in a desiccator with silica gel at the relative humidity of 24% to remove the absorbed moisture until all specimens showed no change in mass. Subsequently, they were placed in a desiccator at the relative humidity of 98%, created by using a saturated solution of K2SO4. Moisture desorption and sorption experiments were performed at room temperature (T = 20 °C) to prevent temperature-induced effects on the absorption process. For the investigation of the moisture absorption kinetics, the specimens were periodically removed from the desiccator, air dried for 5 min, and then weighed by using a Mettler Toledo XS205DU balance (accuracy: 0.05 mg). A period of 5 min was sufficient to remove the surface moisture, and no significant desorption from the bulk of specimens was observed. The results obtained in the moisture absorption experiments were accurately compared for
Notably, two states of water can be observed in polymer-based systems upon moisture absorption: (1) unbound free water, which fills the nanovoids (also called free volume), not inducing swelling; (2) hydrogen-bounded water, which causes swelling of the polymers [3,10]. Composites with a high content of free volume have considerable space available for diffusing water molecules, resulting in high moisture uptake; this, in turn, may enhance the negative effect of moisture on the mechanical properties of composite systems [5]. In FRP systems, moisture is introduced via diffusion flow along the fibre/matrix interface, or conveyed via microcracks and voids, leading to the diffusion in the surrounding matrix [3]. Thus, when MWCNTs are well dispersed within the epoxy, the reduction in the free volume may cause significant improvements in the sorption and mechanical characteristics of the composite system. The moisture absorption curves obtained for EP, NC, CFRP, and CFRP-NC are given in Fig. 1. The moisture content w(t) was defined as the mass difference between moistened mt (time-varying) and initial m0 specimens, normalized to the initial mass of specimens according to the relation:
wðtÞ ¼
mt m0 100%: m0
ð1Þ
For the description of moisture absorption curves of epoxies and epoxy-based composites (including CFRP), classical second Fick’s law is usually applied [10–12]. In the case of the 1D diffusion
Fig. 1. Moisture uptake by epoxy resin (s), nanocomposite (NC) (d), carbon fibre reinforced plastic (CFRP) (e), and CFRP impregnated with NC (r). Lines are calculated by using Eq. (2).
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mode (when the thickness of the test specimens is significantly smaller than the length and width), the moisture content is expressed as follows:
wðtÞ ¼ w1
1 2ðw1 w0 Þ X ð1 ð1Þk Þ
p2
k¼1
2 ! pk exp Dt : h
k
2
2
ð2Þ
Here, w0 and w1 are the initial and equilibrium moisture contents in the specimens, respectively, while h and D are the thickness and diffusion coefficient of the specimens, respectively. As seen from Fig. 1, Fick’s model was in good agreement with the experimental data for all tested materials (i.e., EP, NC, CFRP, and CFRP-NC). For CFRP and CFRP-NC, the sorption process was slower, and the level of moisture uptake was lower than that of the EP and NC systems. This was due to the presence of a significant content (60 vol%) of moisture-impenetrable carbon fibres and, consequently, to the small amount of free volume in FRP available for water molecules diffusing through the test specimens. The data scattering from the averaged values of moisture content was moderate, and a standard deviation of less than 10% was observed. EP specimens had the most scattered data, possibly due to the manual mixing process and the consequent appearance of different inhomogeneities in the material (areas of different density, pores, etc.). In turn, NC samples showed a minor standard deviation from the averaged value of the moisture content. The low values of standard deviation revealed the homogeneous dispersion of MWCNTs in the epoxy as well as their compatibility with EP [13]. Classical Fick’s model is easy to use as it contains only two parameters: the diffusivity D and equilibrium moisture content w1. When the sorption curve is given on a diagram with abscissa p axis t (see Fig. 1), the initial section of this diagram is a line passing through the origin of the coordinates. Thus, by using experimental data, the diffusivity can be determined from its inclination:
D¼
ph2 wðtÞ w0 16t
w1 w0
2 :
ð3Þ
Generally, the equilibrium moisture content w1 is experimentally found as the maximal moisture content observed in the specimen. For all tested materials, the parameters of the sorption process, i.e., the diffusivity found according to Eq. (3) and equilibrium moisture content, are shown in Fig. 2. As shown in Fig. 2, the sorption process observed for the NC was slower than that seen for the EP. The NC diffusivity decreased almost by one third (31%), compared with that of the EP. In addition, the equilibrium moisture content of the materials investigated decreased by 15% in the case of NC, whereas it remained
Fig. 2. Diffusivity D (left ordinate axis) and equilibrium moisture content w1 (right ordinate axis) of epoxy resin, nanocomposite (NC), carbon fibre reinforced plastic (CFRP), and CFRP impregnated with NC.
almost identical (reduced by 0.47%) for CFRP-NC. This effect could be associated with the aspect ratio of the MWCNTs, i.e., 30–80, which led not only to an increased tortuosity of the path of the water molecules upon moisture diffusion into the diffused system, but also to a restriction of the molecular dynamics for the polymer chains surrounding the nanoparticles, causing a retardation of their relaxation. The improved sorption characteristics also indicated the good adhesion between EP and MWCNTs in NC, and MWCNTs and carbon fibres in CFRP-NC systems [5]. For the CFRP-NC, the retarded relaxation of the epoxy chains was less dominant, owing to the rigid network of carbon fibres. Therefore, the effect of the MWCNTs on the sorption characteristics was less prominent in CFRP-NC. Notably, such a reduction in the diffusivity was also characteristic to other kinds of nanoparticles having high aspect ratio. Similar results were obtained previously by the authors for epoxy filled with different contents of montmorillonite clay [14]. Clearly, besides the different chemical composition, montmorillonite clay has a layered structure, and the most pronounced reduction in the moisture diffusion process was obtained in the case of fully exfoliated clay platelets. Nevertheless, the analysis of the MWCNT dispersion in the epoxy [8] confirmed that the MWCNTs formed microsized agglomerates, likely minimizing the prominent effects of the sorption properties of the NC and CFRP-NC systems. The improved barrier characteristics for polymers filled with nanoparticles still offer a possibility to achieve improvements also in the long-term deformability of such materials. To this end, further investigations and discussions should be conducted. 3.2. Effect of moisture on the elastic flexural properties of EP, NC, CFRP, and CFRP-NC The representative stress-strain curves for three-point bending tests of four different materials (EP, NC, CFRP, and CFRP-NC) in the ‘‘dry state” were fully discussed in [8]. In this section, the attention is focused on the effects induced by the absorbed moisture on the elastic flexural properties of these materials. Flexural strength was defined as the maximal value of stress achieved by the specimens. The flexural modulus was calculated from the slope of a secant line between 0.05% and 0.25% strains on the stress–strain plot. The representative stress–strain curves for EP, NC, CFRP, and CFRP-NC in dry and moistened states are given in Fig. 3a. For all materials, the relative changes in flexural strength and modulus, due to absorbed moisture, are shown in Fig. 3b. In contrast to dry materials, for which only insignificant ‘‘nanoeffects” on the elastic flexural properties were observed [8], moistened NC and CFRP-NC revealed improved resistance to moisture-induced reduction in both flexural characteristics. As seen in Fig. 3b, the flexural strengths for moistened EP, NC, CFRP, and CFRP-NC decreased by 16%, 8%, 12%, and 6%, respectively. Slightly higher variations of 29%, 21%, 18%, and 6%, were recorded for the flexural moduli, respectively. Thus, the addition of MWCNTs to EP and CFRP led to an overall reduction in the detrimental effect of moisture absorption on these characteristics. The decrease in the flexural strength of NC and CFRP-NC was half that of the EP and CFRP systems, whereas the flexural modulus was 1.4 and 3 times lower than that of the EP and CFRP systems. As for the sorption characteristics, such results can be attributed to the reduction in the free volume in the composite system by the addition of MWCNTs. This resulted in the reduced moisture absorption and subsequent mitigation of the negative effect of the absorbed moisture. The improved mechanical properties of the moistened NC and CFRP-NC systems, due to the addition of MWCNTs, also confirmed that the adhesion between epoxy resin, MWCNTs, and carbon fibres was sufficiently good, ensuring better stress transfer than that of EP and CFRP. Nevertheless, the environ-
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Fig. 3. Representative stress-strain curves of epoxy resin (solid lines to the right), nanocomposite (NC; dashed lines to the right), carbon fibre reinforced plastic (CFRP; solid lines to the left), and CFRP impregnated with NC (dashed lines to the left) in dry (1) and moistened (2) states (a), and the effect of absorbed moisture on relative change of flexural strength and modulus (b).
mental exposure resulted in reduced interfacial stress transmissibility due to matrix plasticization, chemical changes, and mechanical degradation in the interphase/interface area [3]. Notably, the results obtained for the tensile tests of the epoxy/ MWCNT composites after hydrothermal ageing [15] showed that the plasticization ability of the absorbed moisture resulted in almost the same reduction in the elastic modulus and strength (5–8% and 18–22%, respectively). This was attributed to the altered internal stress conditions and debonding due to the volume expansion among nanoparticles and polymer matrix [15,16]. However, although the effect of moisture on the elastic flexural properties of EP and CFRP was relevant, it could be mitigated by the addition of MWCNTs. These results could be further exploited for applications that involve long-term use and require stable mechanical properties in natural environments with relevant concentration of humidity or aggressive liquid, e.g., transportation, construction, etc. 3.3. Effect of moisture on the viscoelastic flexural properties of EP, NC, CFRP, and CFRP-NC Moisture, similarly to stress and temperature, leads to the acceleration of creep processes in materials. Flexural creep experiments were performed in four cycles with step-by-step increasing stress. The effect of the nanofiller was expected to be more noticeable at the highest values of stress and moisture content. The effect of cycling was investigated as in [8] by comparing the main strain components (elastic, viscoelastic, and plastic) for all tested materials in dry and moistened states.
Fig. 4. Representative creep curves for epoxy resin (solid lines) and nanocomposite (dashed lines) in dry (1) and moistened (2) states at the highest stress level (0.75% of flexural strength).
The creep curves for EP and NC samples in dry and moistened states at the highest stress levels are shown in Fig. 4. Obviously, absorbed moisture essentially acted as plasticizer for the system, leading to an increase in deformability. The addition of MWCNTs to the EP system reduced such a negative effect associated with the moisture absorption. In fact, the NC and CFRP-NC samples had lower values for all creep strain components than the EP and CFRP samples (see Fig. 5). Thus, the nanomodified materials exhibited improved creep resistance in the moistened state. The addition of MWCNTs to EP and CFRP resulted in almost equivalent improvements in creep resistance in relation to the stress applied: the elastic strain was reduced by 1.25 and 1.04 times, viscoelastic strain by 1.39 and 1.03 times, and plastic strain by 2.68 and 1.60 times, respectively. The reduction in viscoelastic and plastic creep strains for both NC and CFRP-NC specimens could be attributed to the removal of MWCNTs from the agglomerates and subsequent destruction of the agglomerates. This occurred upon cyclic and increasing loadings, leading to improved matrix–filler interfacial interactions [8]. The creep strain analysis, conducted for all materials in dry (e0) and moistened states (ew), allowed investigating the relative moisture-induced effect (ew/e0) at the highest value of applied stress (Fig. 6). For EP and NC, the effect of moisture, normalized to the creep strain of the dry state, resulted in the increase at the mean: by 1.33 and 1.32 times for elastic strain, 1.48 and 1.24 times for viscoelastic strain, and 1.53 and 1.28 times for plastic strain, respectively. For CFRP and CFRP-NC, the effect was quite insignificant for elastic strain—increase at the mean of 1.09 and 1.03 times, respectively—and more pronounced for viscoelastic—increase at the mean by 1.52 and 1.33 times, respectively—and plastic—increase at the mean by 1.45 and 1.39 times, respectively—strains. Nevertheless, the values of viscoelastic and plastic strains for CFRP were so small (<0.007%) that such effect can be neglected. Notably, the addition of MWCNTs resulted in a reduction in the effect of moisture on creep strains, due to the retardation of the relaxation processes upon creep in all considered cases. Thus, the addition of MWCNTs promoted the formation of restriction sites for relatively slow regrouping of polymer macromolecules upon creep. This result provides the possibility to improve the creep resistance for both epoxy and CFRP not only in the dry state, but also in the moistened state. The time-stress superposition principle and the Boltzmann– Volterra linear integral equation can be used for the description of a series of creep curves at various stresses for EP and NC in the moistened state. The master curves of creep compliance for the materials in the moistened state were compared with those obtained for the materials in the dry state (Fig. 7).
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Fig. 5. Effect of moisture on elastic ee (a), viscoelastic eve (b), and plastic ep (c) strains of moistened epoxy resin (e) and nanocomposite (r) (NC; left ordinate axis), and carbon fibre reinforced plastic (CFRP) (h) and CFRP impregnated with NC (j) (right ordinate axis).
Fig. 6. Creep strains for epoxy resin, nanocomposite (NC), carbon fibre reinforced plastic (CFRP), and CFRP impregnated with NC in moistened state as normalized to dry state (ee, elastic strain; eve, viscoelastic strain; ep, plastic strain).
Obviously, the master curves for moistened EP and NC were located above the master curves for the same materials in the dry state. Nevertheless, the master curve for moistened NC was positioned under the master curve of moistened EP. This effect increased with time and provided an improvement in the longterm creep resistance of MWCNT-filled epoxy. Consequently, the operation time of such materials will be longer under both dry and humid conditions. The results obtained for the time-stress shift factor (Fig. 8) for moistened epoxy and NC completed the above analysis of the improved creep resistance due to the addition of MWCNTs. It should be noted that the standard deviation in Fig. 8 for the time-stress shift factor of moistened NC at the highest stress levels was only 0.05, and was located inside the markers. Evidently, the time-stress shift factor for the nanomodified epoxy specimens,
Fig. 7. Master curves for epoxy resin and nanocomposite in dry (s and D) and moistened (d and ▲) states, obtained using the time-stress superposition principle.
having absorbed moisture, was reduced at all applied stresses. These results provide the possibility to expand the use of such materials to different applications that involve long-term use and require stable mechanical properties under humid conditions.
4. Conclusions Short-term cyclic creep-recovery tests of moistened EP, NC, CFRP, and CFRP-NC showed the reduction in all creep strains in nanomodified materials (NC and CFRP-NC). The improvement in creep resistance could be explained by considering the decreased mobility of polymer chains both in dry and moistened states, as the MWCNTs acted as restriction sites. On the one hand, the addition of moisture-impenetrable MWCNTs to the epoxy resin caused the retardation of moisture dif-
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Acknowledgments The research of A. Aniskevich was supported by the Latvian state research programme under grant agreement ‘‘IMATEH”. References
Fig. 8. Time-stress shift factor vs difference in applied stress, starting from the stress of the first cycle for epoxy resin and nanocomposite in dry (-s- and -h-) and moistened (-d- and -j-) states, respectively.
fusion, resulting in the reduction in the diffusion coefficient by 31% and equilibrium moisture content by 15%. Minor effect on the same sorption characteristics was found for epoxy-based CFRP. On the other hand, MWCNTs, having high stiffness and strength, improved the flexural properties of these materials: the flexural strengths of NC and CFRP-NC were reduced twice, the flexural moduli by 1.4 and 3 times, respectively, while the creep strains were reduced by 1.25 and 1.04 times (elastic), 1.39 and 1.03 times (viscoelastic), and 2.68 and 1.60 times (plastic), respectively. The improved mechanical properties of the moistened NC and CFRP-NC showed that the adhesion between the epoxy resin, MWCNTs, and carbon fibres was sufficiently good, and ensured a superior stress transfer, compared with that of EP and CFRP. Clearly, the moisture-induced effects that led to such significant reduction in the mechanical characteristics were crucial. This was ascribed to the reduced interfacial stress transmissibility due to matrix plasticization, chemical changes, and mechanical degradation in the interphase/interface area. Further improvements in barrier characteristics and stability against environmental factors can be obtained by optimization of the dispersion process and chemical functionalization of MWCNTs. In this way, the agglomeration of nanoparticles can be reduced, and the interfacial strength between them and the polymer matrix can be improved. Thus, owing to the addition of stiff and moisture-impenetrable MWCNTs, the negative effect of absorbed moisture was reduced. As a result, MWCNT-filled CFRPs can be used as structural components in different applications requiring high performance under humid conditions.
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