- Email: [email protected]
Covalent functionalization of aramid fibers with zinc oxide nano-interphase for improved UV resistance and interfacial strength in composites
Composites Science and Technology 188 (2020) 107996
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Covalent functionalization of aramid fibers with zinc oxide nano-interphase for improved UV resistance and interfacial strength in composites Lixiang Ma, Jingwei Zhang, Cuiqing Teng * State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, 201620, PR China
A B S T R A C T
Improving the resistances of organic high-performance fibers to harsh environments and enhancing the interfacial interactions of fiber-reinforced composites have become crucial in various applications. In this report, ZnO nanoparticles (NPs) and ZnO nanowires (NWs) were successfully “grown” on the surfaces of aramid fibers (AFs) by grafting with γ-aminopropyl triethoxysilane (KH550) followed by the growth of nano-ZnO. The surface functionalized AFs exhibited improved UVresistances. After 168 h of ultraviolet exposure, the tensile retention rates of the ZnO-NP- and ZnO-NW-grafted AFs reached 95.6% and 97.7%, respectively, which were significantly higher than the value of 79.1% of the bare fiber. Meanwhile, the introduction of the KH500 and ZnO formed a nano-interphase, enhancing the interfacial strength of the fiber-reinforced epoxy resin composites. The interfacial shear strengths (IFSSs) of the composites with AF-g-ZnO NPs and AF-g-ZnO NWs were 42.9 and 47.8 MPa, respectively, whereas that of bare AF-reinforced epoxy resin was only 31.2 MPa. Nano-ZnO was physically deposited on the AF surfaces without KH550. The IFSS was 36.4 MPa for the AF-ZnO NP and 38.8 MPa for the AF-ZnO NW, which were lower than those of the grafted composites. Therefore, the chemical grafting nano-ZnO on high-performance fibers provides a new strategy for improving the UV-resistances of advanced fibers and to enhance the mechanical properties of fiber-reinforced composites.
1. Introduction Aramid fibers (AFs), such as Kevlar®, Technora® or Rusar®, exhibit superior performances, including high thermal stabilities, good chemi cal resistances, and excellent specific tensile strengths. Thus, fiberreinforced polymer composites have been extensively used in various fields, including aerospace, the military, the automotive industry, and the energy industry [1]. As a high-performance fiber, the amide-bond structures of AFs are easily broken under UV light, causing aging degradation of the fiber when it is exposed to harsh environmental conditions with high temperatures and ultraviolet (UV) radiation [2–5]. To prevent the loss of mechanical properties, methods to improve the UV resistance have been evaluated, such as coating a resin or metal oxide on the fibers [6–8]. At present, the main method to improve the UV resis tance is grafting a UV absorber on the AFs. Organic UV absorbers generally have poor thermal resistances and limited lifespans, whereas inorganics can indefinitely absorb UV radiation due to their inherent structures. ZnO is an excellent UV absorber, and ZnO nanoparticles can be coated on the surfaces of fibers to improve their UV resistances in nature. However, there are not strong chemical interactions between ZnO and fibers, resulting in debonding at the interfacial phase, thereby leading to the deterioration or even failure of the composites.
It is well known that the mechanical properties of fiber-reinforced polymer composites are influenced by the interfacial characteristics between the fiber and resin matrices. However, the surfaces of the AFs are chemically inert and smooth due to the high crystallinities of the fiber surface layers and the lack of polar functional groups in the macromolecular chains, resulting in a poor adhesion between the AFs and the resin matrix [9,10]. Therefore, surface modification is essential to enhance the interfacial bonding strengths of AFs. Various approaches for surface modification have been developed to reinforce the interfacial interactions between fibers and matrices, such as chemical etching [11, 12], grafting [13–15], plasma treatment [16], γ-ray radiation [17], electrochemical oxidation [18], and various other treatments [19]. Among them, the chemical modification approaches are convenient, stable, and suitable for industrial batch processes, forming stable chemical bonds between the fiber and matrix via chemical reactions. For instance, the interfacial shear strength interfacial shear strength (IFSS) of poly(p-phenylene-benzimidazole terephthalamide) (PBIA) fiber and epoxy was improved by nearly 27% by grafting 4-(bromomethyl) ben zoic acid on the fiber [20]. One promising approach for the fabrication of hybrid composites is the growth of inorganic nanowire arrays on the fiber surface to provide improved interfacial strength and out-of-plane reinforcement. As reported by Sodano et al. in 2009 [21,22], a novel
* Corresponding author.; E-mail address: [email protected] (C. Teng). https://doi.org/10.1016/j.compscitech.2020.107996 Received 16 April 2019; Received in revised form 5 November 2019; Accepted 5 January 2020 Available online 10 January 2020 0266-3538/© 2020 Elsevier Ltd. All rights reserved.
L. Ma et al.
Composites Science and Technology 188 (2020) 107996
functionalization method for aramid or carbon fibers was developed to enhance the bonding of a ZnO nanowire interphase grown on the fiber surface for interfacial strength enhancement. Moreover, they controlled the morphologies and arrays of the nanowires, which had significant effects on various properties, including the interfacial interactions, impact properties, and interyarn friction [23–27]. As mentioned above, simultaneously overcoming the two major inherent shortcomings of AFs has attracted widespread attention in in dustry and academia. In this study, the commercial coupling agent γ-aminopropyl triethoxysilane (KH550) was used to modify the surfaces of AFs by chemical grafting, and fiber-grafted Si–OH was successfully prepared. The ZnO nanoparticles containing hydroxyl groups were subsequently grafted on the surfaces that were modified by a dehydra tion reaction, and ZnO nanowires further “grew” from the active seeds of the ZnO nanoparticles on the surfaces of the AFs. The functionalized AFs exhibited improved UV resistance and enhanced interfacial strength between the fibers and matrices. Thus, the chemical grafting of nanoZnO on high-performance fibers provides a new method for over coming the crucial issues of UV resistance and interfacial interactions.
2.2. Surface functionalization of AFs The functionalization of the aramid fiber included hydrolysis treat ment with alkali followed by acidification, surface treatment with acyl chloride and KH550, the grafting of ZnO nanoparticles, and the growth of ZnO nanowires (NWs), as illustrated in Fig. 1. The aramid fibers were washed successively in acetone and ethanol solutions to the remove surface sizing agent, after which the fiber was rinsed with deionized water and dried. A common hydrolysis process was used to cleave the peptide bonds between the amide and carbonyl functional groups that are inherent to the aramid structure [21,28]. First, the fiber was immersed in a 30 wt% NaOH aqueous solution for 30 min, after which it was washed three times with water and immersed in a 33% HCl aqueous solution for 30 s. The carboxyl functionalized aramid fibers (AF-COOH) was thereby obtained after washing with deionized water and drying in a vacuum at 120 � C for 1 h. Second, the AF-COOH fibers were further modified chemically with a solution of 100 mL of SOCl2 and 5 mL of DMF at 30 � C for 72 h to obtain acyl chloride functionalized aramid fibers (AF-COCl). The AF-COCl fibers were subsequently washed with tetrahydrofuran (THF) and immersed into a THF solution containing 5 wt% KH550 at 60 � C for 4 h. With the reaction between the –NH2 groups of the KH550 and the -COCl groups of the above functionalized AFs, Si–OH was introduced to the fiber sur faces. The surface-modified fibers (AF-KH550) were subsequently washed with THF to remove residual KH550 and dried in vacuum at 80 � C for 2 h. A zinc oxide seed solution was synthesized using the method re ported by Wong et al. [29], as follows. Two solutions of 8 mM sodium hydroxide in ethanol and 1.6 mM Zn(CH3COO)2⋅2H2O in ethanol were prepared. A zinc acetate solution was preheated to 60 � C, after which the sodium hydroxide was added with vigorous stirring. After 30 min, the seed solution was quenched in ice water to slow the nanoparticle (NP) growth. The modified aramid fibers (AF-KH550) were dipped into the seeding solution at 60 � C for 20 min and annealed at 120 � C for 20 min. This was repeated three times to ensure that the nanoparticles were grafted uniformly onto the fiber surfaces, and thus, the AF-g-ZnO NP sample was obtained. Next, the AF-g-ZnO NP sample was added to the growth solution for the ZnO nanowires at 87 � C for 5 h, which contained 25 mM Zn(NO3)2⋅6H2O, 25 mM HMTA, and 3 mM PEI in deionized water. After 5 h, the fibers were removed, washed with deionized water,
2. Experiments 2.1. Materials Benzimidazole-contained aramid fiber (AF) used in this research was supplied from Chengrand Research Institute of Chemical Industry Co. Ltd., whose chemical structure is similar to Rusa® of poly(p-phenylenebenzimidazole terephthalamide) (PBIA). The filament diameter was 17 μm, and the tensile strength of the fiber is 5.23 GPa. Analytical grade regents of zinc acetate dihydrate (Zn(CH3COO)2⋅2H2O, 99.0%), zinc nitrate hexahydrate (Zn(NO3)2⋅6H2O, 99.0%) and polyethyleneimine (PEI, Mn ¼ 600, 99.0%) were obtained from Aladdin Co., Ltd. N,Ndimethylformamide (DMF), tetrahydrofuran (THF), thionyl chloride (SOCl2, 99.0%), sodium hydroxide (NaOH, 99.0%) and hexamethy lenetetramine (HMTA, 99.9%) were supplied from Sinopharm Chemical Reagent Co., Ltd. γ-Aminopropyl triethoxysilane (KH550) was supplied by Sigma-aldrich (Shanghai) Trading Co., Ltd. Two-component resin, including epoxy resin WSR618 (E51) and phenolic aldehyde amine hardener (T31), was obtained from Sinopharm Chemical Reagent Co., Ltd.
Fig. 1. Schematic of the preparation of the AF-g-ZnO NWs. 2
L. Ma et al.
Composites Science and Technology 188 (2020) 107996
and dried at 120 � C. The fibers obtained using KH550 were denoted as AF-g-ZnO NW, and those without KH550 were denoted as AF-ZnO NW. 2.3. Characterization Attenuated total reflectance-infrared spectroscopy (ATR-IR) was performed on a Nicolet 8700 spectroscope. X-ray photoelectron spec troscopy (XPS) was carried out on an Escalab 250Xi using a mono chromatic Al X-ray source (987.9 W, 1486.6 eV), and all samples were analyzed using Avantage software for peak fitting and integration. The structures of samples were analyzed using X-ray diffraction (XRD) with D/max-2550 (Rigaku, JAN). The size distributions of the ZnO nano particles were measured using a particle size analyzer (Zetasizer Nano ZS, UK). The surface morphologies of the fibers were observed using field-emission scanning electron microscopy (SEM, Hitachi SU8010, JAN), and the morphologies of the ZnO nanoparticles were observed using transmission electron microscopy (TEM, JEM-2100F, JAN). Tapping-mode atomic force microscopy (AFM) measurements were obtained using a Dimension FastScan Bio (Bruker, USA). Ultraviolet absorption spectra of the samples were obtained using a Lambda 45 (PE, USA). The mechanical properties were measured using an XQ-1 tensile testing instrument with a gauge length of 20 mm at a strain rate of 1% s 1. For each fiber sample, at least 15 filaments were tested. UV radia tion tests were conducted by exposing the fibers to a UV light source with a wavelength range of 290–365 nm at 3 kW for 168 h and a relative humidity of 60% using an instrument manufactured by the Nanjing Wuhe Test Equipment Co. Ltd. (China). Single-fiber microdroplet tests were performed to measure the IFSSs of the fibers modified with an epoxy matrix. The fibers were embedded in an epoxy mixture of the WSR618 epoxy resin and a phenolic aldehyde amine hardener T31 with a weight ratio of 10:3. Single-fiber pull-out testing was performed on an XQ-1 tensile testing instrument, and the size of the droplet was measured using optical microscopy (Lecia DM2500P). The IFSS was calculated using the following equation [30]:
τIFSS ¼
F
π df L
Fig. 2. ATR-FTIR spectra of AF, AF-COOH, AF-COCl, and AF-g-KH550.
reaction was presumed to occur mainly on the surface, the intensity of the fiber bulk was so high that the tiny change on the surface almost was ignored. Moreover, the band around 675 cm 1 was assigned to the C–Cl stretching vibrations of acid chloride, as evident for the AF-COCl and AF-g-KH550 samples. For the KH550-grafted AF, the absorption peaks at 2937 and 2869 cm 1 were attributed to asymmetric and symmetric stretching vibrations, respectively, of methyl and methylene groups. The peaks at 1110 and 1018 cm 1 were attributed to Si–O stretching vi bration and Si–O–C stretching vibrations, respectively [31]. The signif icant enhancement of the absorption peak at 1032 cm 1, which was attributed to the Si–O–C stretching vibrations, proved that KH550 was successfully grafted to the AF surface via covalent bonds. To further investigate the surface structures of the fibers with various treatments, XPS spectra of the AFs, AF-COOH, AF-COCl, AF-g-KH550, and AF-g-ZnO were obtained, as shown in Fig. 3, and the correspond ing elements are listed in Table 1. After hydrolysis treatment, no sig nificant changes in the C and N content were evident, except for a slight increase in the O content. The presence of Cl, Si, and Zn in the AF-COCl, AF-g-KH550, and AF-g-ZnO samples indicated that the AFs were suc cessfully modified by the acid chloride, KH550, and grafted ZnO NPs. Fig. 4 shows the C1s core-level spectra of the functionalized fibers. The C1s core-level spectrum of the pristine AFs was curve-fitted with four peak components, with binding energies (BE’s) at 284.6 eV for C–C, – N, and 288.0 eV for O– – C–NH. In 285.4 eV for C–N, 286.4 eV for N–C– – O (288.9 eV) and Si–C Fig. 4(b) and (c), with the emergence of the O–C– (284.1 eV) peaks, the formation of COOH produced by amide hydrolysis and the grafting with KH550 was evident, confirming that KH550 was
(1)
where F is the tensile force at debonding, df is the diameter of the tested fiber, and L is the embedded length of the epoxy microdroplet. Twenty samples were tested, and the average value was computed. 3. Results and discussion 3.1. Graft reaction on AF surface Directly coating nano-ZnO on the fibers was difficult due to the weak adhesion between the inorganic nanowires and the organic aramid fi bers, and the coating easily fell off the surfaces of the fibers. To improve the interaction of the ZnO nanowires with the highly crystalline aramid fibers, a fiber surface functionalization was performed, as reported by Sodano et al. [21,22]. We further modified the surface functionalization using a silane coupling agent (KH550). Fig. 1 illustrates the detailed procedure for preparing functionalized aramid fibers (AF-g-ZnO), in which nano-ZnO was grafted to fiber surfaces due to the KH550. The schematic of the preparation of the AF-g-ZnO consisted of four steps: hydrolysis treatment of AFs with alkali followed by an acidification modification, surface treatment with acyl chloride, introduction of sili con–oxygen bonds due to the KH550, grafting of the ZnO nanoparticles, and the growth of ZnO nanowires. Fig. 2 shows the ATR-FTIR spectra of the untreated AFs, hydrolyzed AFs (AF-COOH), acylated AFs (AF-COCl), and KH550-grafted AFs (AF-g– O stretching peak at 1640 cm 1, KH550). For the untreated AFs, the C– 1 N–H bending peak at 1545 cm and C–N stretching peak at 1310 cm 1 were evident. Apparently, the sodium hydroxide treatment resulted in a slight decrease in the relative intensity of the C–N absorption. Since the
Fig. 3. Wide-scan XPS spectra of the AFs, AF-COOH, AF-COCl, AF-g-KH550, and AF-g-ZnO NPs. 3
L. Ma et al.
Composites Science and Technology 188 (2020) 107996
Zetasizer Nano ZS. As shown in Fig. 6(b), the average sizes of the par ticles was 12.8 � 1.45 nm, which was in good agreement with the value obtained by TEM. Particles with a narrow distribution were uniformly coated on the fiber surface, limiting the formation of defects that could weaken the interface. In addition to the uniformity of the nanoparticles, the crystal structure also affected the strength of the bonding to the fiber surface. Previous studies found that the wurtzite structure was respon sible for the strong intermolecular bonding between the zinc oxide and functional groups on fibers [32,33].
Table 1 Elemental contents of aramid and modified fibers. Sample
C
N
O
Cl
Si
Zn
AF AF-COOH AF-COCl AF-g-KH550 AF-g-ZnO
80.16 79.70 72.89 58.94 53.07
8.13 7.09 7.24 9.03 3.25
11.71 13.21 16.06 23.93 32.3
– – 3.81 0.57 –
– – – 7.53 3.12
– – – – 8.26
successfully grafted onto the AF surfaces by covalent bonds, which was consistent with the ATR-FTIR results. Fig. 5 provides the XRD patterns of the pristine AFs, AF-COOH, AFCOCl, AF-g-KH550, AF-g-ZnO, and ZnO NPs. All the fibers exhibited similar patterns at 2θ ¼ 20� , corresponding to the AF crystallinity, which demonstrated that the modification processes, including hydrolysis, acidification, and acylation, did not damage the crystal structure of the AFs. A small diffuse peak appeared at approximately 5.3� due to longrange order in the AF-g-KH550 and AF-g-ZnO. This suggested that KH550 was grafted to the AFs due to the dehydration reaction of the –OH groups of KH550 and nano-ZnO. The diffraction pattern of ZnO confirmed that the wurtzite crystal structure was present, according to the JCPDS (no. 36–1451). The AFs grafted with ZnO NPs with KH550 also showed a corresponding crystal structure. 3.2. Surface morphologies of fibers Fig. 6(a) shows TEM images of the ZnO particles grown from Zn (CH3COO)2 at 55 � C for 0.5 h. The particles were spherical approxi mately from the lattice stripes, with diameters of ~10 nm. The sizes and distribution of the particles in the colloidal solution were measured on
Fig. 5. XRD patterns of the AFs, AF-COOH, AF-COCl, AF-g-KH550, AF-g-ZnO, and ZnO NP.
Fig. 4. C1s core-level of pristine AFs, AF-COOH, AF-COCl, and AF-g-KH550. 4
L. Ma et al.
Composites Science and Technology 188 (2020) 107996
Fig. 6. (a) TEM image (b) size distribution of ZnO NPs.
The surface modification caused remarkable changes in the surface topographies of the fibers, as shown in Fig. 7. The surface of a pristine AF (Fig. 7(a)) exhibited an extremely smooth surface with tiny grooves that resulted from the spinning process, and a similar morphology was observed in the AF-COOH (Fig. 7(b)). However, the grooves on the surface of the AF were significantly deepened due to the acylation treatment (Fig. 7(c)). At a high magnification (inset), an extremely thin polymer coating with some small spots was observed on the surface of the AF-g-KH550 (Fig. 7(d)). The AF-g-ZnO NPs were evident in the top-
down images of the AF-grafted ZnO NPs (Fig. 7(e) inset). The surface of the AF was uniformly covered with nanoparticles, resulting in fewer defects and a rougher surface. Further, the growth of the ZnO NW array was more regular. As shown in Fig. 7(f), ZnO NWs grew vertically and uniformly on the AF surface, and the nanowires had an average length of 1–2 μm. AFM images further confirmed the surface topographies of the functionalized fibers. As shown in Fig. 8, the untreated AF had a few relatively neat shallow grooves with a small roughness of 2.35 nm. No
Fig. 7. SEM images of (a) untreated AF, (b) AF-COOH, (c) AF-COCl, (d) AF-g-KH550, (e) AF-g-ZnO NP, and (f) AF-g-ZnO NW. 5
L. Ma et al.
Composites Science and Technology 188 (2020) 107996
original morphology, and there was no damage on the fibers grafted with ZnO, as shown in Fig. 10(c) and (d). The nano-ZnO functionaliza tion protected the outer surface of the AFs from the surface etching caused by UV irradiation, resulting in less deterioration of the tensile strength.
changes were evident on the surface of the AF-COOH (3.42 nm) relative to the AF. Fig. 8(c) indicated that the AF-COCl surface became much rougher (7.83 nm) than that of the AF-COOH, which was in good agreement with the SEM image of the AF-COCl. The KH550-grafted AFs exhibited many spots on the surfaces of the fibers with a significantly increased roughness (16.51 nm), as shown in Fig. 8(d). When the ZnO NPs were grafted to the surfaces of the fibers, many more tiny spots appeared, corresponding to a roughness of 21.44 nm. The rougher sur face morphology could not only increase the contact area but also strengthen the mechanical interlocking between the fiber and resin matrix. To study the role of the KH550 in the interphase between the aramid fibers and ZnO NWs, a sample of the AF-ZnO NWs without KH550 was prepared, as described in the Experimental section. The fibers of the AFg-ZnO NW and AF-ZnO NW were destroyed. As shown in Fig. 9, the ZnO NW coating without KH550 peeled off the aramid fiber in the interphase region. On the contrary, KH550 modified ZnO NWs firmly adhered to the AF surface, forming a strong nano-interface. The root of the nano wires were tightly bound to the surface of the fiber, and the debonding behavior did not occur when the interface was damaged in the AF-g-ZnO NW sample. The above results showed that the ZnO grafting with KH550 did not hinder the growth of the ZnO NWs on the fiber surface but significantly improved the adhesion of the ZnO to the fibers instead.
3.4. Interfacial adhesion of fibers/epoxy composites Micro-debonding tests were carried out to assess the effect of the ZnO graft modification on the interfacial adhesion of the composites. Two methods to prepare the modified fibers were compared, i.e., physical deposition and chemical grafting methods. The AFs modified using a ZnO physical deposition method without KH550 were named AF-ZnO NP and AF-ZnO NW, and the AFs modified by ZnO chemical grafting with KH550 were named AF-g-ZnO NP and AF-g-ZnO NW. AF-reinforced epoxy composites were prepared, and the interfacial shear strength (IFSS) results are shown in Fig. 11. For the composites reinforced by the bare AF, the IFSS was 31.2 MPa, and the debonding section (Fig. 12 (b)) was very smooth, indicating that there was weak adhesion between the fibers and epoxy resin. With the physical deposition of the ZnO NPs on the AF surface, the IFSS increased by 16.7% compared to that of the bare AF composites. The further growth of the ZnO NWs resulted in a slight enhancement in the IFSS to 38.8 MPa. However, the ZnO NPs easily fell off the surfaces of the fibers, and debonding occurred between the fibers and ZnO, as shown in Fig. 12 (c) and (d). The interfacial properties have an important effect on the bonding of ZnO and fiber. As reported by Sodano et al., for the nano-ZnO/carbon fibers system, ZnO obtains a negative charge after approaching the electron rich surface of oxidized sp2 bonded carbon surface [36], and correlation between interface strength and ketone groups was the highest, due to the limited steric hindrance of the two lone pairs on the oxygen atom [37]. In this work, the chemical grafting method was used to introduce nano-ZnO onto the surfaces of AFs with KH550, and interfacial adhesion between the fiber and resin significantly improved. As shown in Fig. 11, the IFSS values of the composites with AF-g-ZnO and AF-g-ZnO NW increased to 42.9 MPa and 47.8 MPa, respectively, corresponding to 17.8% and 23.2% increases compared to AF-ZnO NP and ZnO NW samples, respectively. Meanwhile, the debonding section underwent significant changes. As shown in Fig. 12(e), the skin of the fiber was completely stripped due to strong shearing effect between the fiber and epoxy resin, and the diameter of the AF-g-ZnO NW after debonding significantly decreased, indicating a strong bonding strength between the fibers and epoxy resin. This reinforcement can be explained by several mechanisms acting on the newly integrated interphase. On the one hand, the nanoparticles or nanowires increased the surface roughness characteristics of the fibers, allowing greater surface in teractions with the wetted epoxy matrix and a small degree of me chanical interlocking. On the other hand, ZnO was grafted to the AF
3.3. UV resistances of fibers ZnO has excellent UV shielding properties, and its shielding effect is mainly achieved by UV absorption and scattering. In the near-UV region, ZnO mainly converts UV irradiation to heat by means of electron exci tation and recombination. Many studies have been conducted to prove that ZnO fillers and films have a protective effect on organics, and the poor UV resistance of AFs is mainly attributed to the amide groups in the fibers, which easily decompose under UV irradiation [34,35]. Over the span of 168 h of UV irradiation, the mechanical properties of pristine and coated AFs were compared, as shown in Table 2. The tensile strength of the pristine fiber was 5.23 GPa, while the value decreased to 4.14 GPa due to the UV irradiation, and the strength retention rate was 79.1%. For the AF-g-KH550, the tensile strength of the fiber without UV irradiation was 4.95 GPa, which was a slight decrease due to the hydrolysis treatment. The value for the UVirradiated fibers decreased to 4.19 GPa, with a strength retention rate of 84.6%. However, for the AF-g-ZnO NP and AF-g-ZnO NW samples, the strength retention rates were 95.6% and 97.7%, respectively, which are significantly different from that of the pristine AF. Apparently, the modification with nano-ZnO significantly improved the UV resistance for the high-performance fibers. The SEM image in Fig. 10(b) shows the damaged surface of the bare fibers due to the severe UV-etching and the creation of vertical cracks. The ZnO-coated fibers maintained the
Fig. 8. AFM images of the surface morphologies of (a) untreated AF, (b) AF-COOH, (c) AF-COCl, (d) AF-g-KH550, and (e) AF-g-ZnO NP. 6
L. Ma et al.
Composites Science and Technology 188 (2020) 107996
Fig. 9. SEM images of the peeled interphases of (A) AF-ZnO NWs and (B) AF-g-ZnO NWs. Table 2 Effects of UV irradiation on tensile strengths of aramid fibers. Samples F3 F3-g-KH550 F3-g-ZnO NP F3-g-ZnO NW
Tensile strength (GPa) No UV irradiation
UV irradiated
5.23 4.95 4.77 4.71
4.14 4.19 4.56 4.60
Strength retention rate (%) 79.1 84.6 95.6 97.7
Fig. 12. SEM images of micro-debonding test samples: (a) micro-debonding sample and (b–f) fracture morphologies after debonding. Fig. 10. SEM micrographs of AFs before and after UV irradiation: (a) AFs before UV irradiation, (b) AFs after UV irradiation, (c) AF-g-ZnO NP after UV irradiation, and (d) AF-g-ZnO NW after UV irradiation.
surfaces by silane coupling agents, which further strengthened the interphase between the fibers and ZnO. Overall, the improved interac tion demonstrated that the ZnO-grafted AFs had stronger interactions with the epoxy matrix compared with the ZnO-deposited AFs. 4. Conclusions Nano-ZnO was uniformly grafted to the fiber surfaces by the “bridge effect” of the silane coupling agent KH550, which significantly improved the UV resistances of the fibers. For instance, after 168 h of UV exposure, the fiber tensile strength increased from 79.1% to 95.6% (AF-g-ZnO NP) and 97.7% (AF-g-ZnO NW). The chemical graft of the ZnO endowed the aramid fibers with high roughness characteristics, which enhanced the interactions between the aramid fibers and ZnO. For the epoxy resin composites reinforced with AF-ZnO NPs or AP-ZnO NWs, the IFSSs were 42.9 and 47.8 MPa, respectively, which were significantly higher than those of the composites without the KH550-grafted AFs. Moreover, based on the SEM observations of the micro-debonding fracture sur faces, a strong interphase for the AF-g-ZnO-reinforced epoxy resin formed, and the interfacial adhesion was significantly better than that of the ZnO-deposited AFs and epoxy resin. Therefore, this report provides an effective method to improve the UV-resistances of advanced fibers
Fig. 11. Interfacial shear strength of the modified AFs and epoxy composites.
7
L. Ma et al.
Composites Science and Technology 188 (2020) 107996
and enhance the mechanical properties of fiber-reinforced composites.
[15] G.J. Ehlert, Y. Lin, H.A. Sodano, Carboxyl functionalization of carbon fibers through a grafting reaction that preserves fiber tensile strength, Carbon 49 (13) (2011) 4246–4255. [16] G.S. Sheu, S.S. Shyu, Surface properties and interfacial adhesion studies of aramid fibres modified by gas plasmas, Compos. Sci. Technol. 52 (4) (1994) 489–497. [17] Y. Zhang, Z. Jiang, Y. Huang, Q. Li, The modification of Kevlar fibers in coupling agents by γ-ray co-irradiation, Fibers Polym. 12 (8) (2011) 1014–1020. [18] M. Andideh, M. Esfandeh, Effect of surface modification of electrochemically oxidized carbon fibers by grafting hydroxyl and amine functionalized hyperbranched polyurethanes on interlaminar shear strength of epoxy composites, Carbon 123 (2017) 233–242. [19] Z. Cheng, C. Chen, J. Huang, T. Chen, Y. Liu, X. Liu, Nondestructive grafting of PEI on aramid fiber surface through the coordination of Fe (III) to enhance composite interfacial properties, Appl. Surf. Sci. 401 (2017) 323–332. [20] Y. Dai, C. Meng, Z. Cheng, L. Luo, X. Liu, Nondestructive modification of aramid fiber based on selective reaction of external cross-linker to improve interfacial shear strength and compressive strength, Compos. Part A 119 (2019) 217–224. [21] G.J. Ehlert, H.A. Sodano, Zinc Oxide Nanowire interphase for enhanced lightweight polymer fiber composites, ACS Appl. Mater. Interfaces 1 (8) (2009) 1827–1833. [22] Y. Lin, G.J. Ehlert, H.A. Sodano, Increased interface strength in carbon fiber composites through a ZnO nanowire interphase, Adv. Funct. Mater. 19 (2009) 2654– 2660. [23] U. Galan, Y. Lin, G.J. Ehlert, H.A. Sodano, Effect of nanowire morphology on interfacial strength of nanowire coated carbon fibers, Compos. Sci. Technol. 71 (2011) 946–954. [24] H.-S. Hwang, M.H. Malakooti, H.A. Sodano, Tailorable interyarn friction of ZnO nanowire-aramid fiber composite fabrics by controlling morphology of ZnO nanowires, Compos. Part A 76 (2015) 326–333. [25] M.H. Malakooti, H.-S. Hwang, H.A. Sodano, Morphology-controlled ZnO nanowire arrays for tailored hybrid composites with high damping, ACS Appl. Mater. Interfaces 7 (1) (2015) 332–339. [26] M.H. Malakooti, H.-S. Hwang, N. Goulbourne, H.A. Sodano, Role of ZnO nanowire arrays on the impact response of aramid fabrics, Compos. Part B 127 (2017) 222–231. [27] H. Hwang, H.M. Mohammad, B.A. Patterson, H.A. Sodano, Increased interyarn friction through ZnO nanowire arrays grown on aramid fabric, Compos. Sci. Technol. 107 (2015) 75–81. [28] E.G. Chatzi, S.L. Tidrick, J.L. Koenig, Characterization of the surface hydrolysis of Kevlar-49 fibers by diffuse reflectance FTIR spectroscopy, J. Polym. Sci. Phys. 26 (8) (1988) 1585–1593. [29] E.M. Wong, J.E. Bonevich, P. Searson, Growth kinetics of nanocrystalline ZnO particles from colloidal suspensions, J. Phys. Chem. B 102 (40) (1998) 7770–7775. [30] B. Miller, P. Muri, L. Rebenfeld, A microbond method for determination of the shear strength of a fiber/resin interface, Compos. Sci. Technol. 28 (1) (1987) 17–32. [31] R. Sa, Y. Yan, Z. Wei, L. Zhang, W. Wang, M. Tian, Surface modification of aramid fibers by bio-inspired poly(dopamine) and epoxy functionalized silane grafting, ACS Appl. Mater. Interfaces 6 (23) (2014) 21730–21738. [32] G.J. Ehlert, U. Galan, H.A. Sodano, Role of surface chemistry in adhesion between ZnO nanowires and carbon fibers in hybrid composites, ACS Appl. Mater. Interfaces 5 (3) (2013) 635–645. [33] B.A. Patterson, U. Galan, H.A. Sodano, Adhesive force measurement between Hopg and zinc oxide as an indicator for interfacial bonding of carbon fiber composites, ACS Appl. Mater. Interfaces 7 (28) (2015) 15380–15387. [34] H. Zhao, R.K. Li, A Study on the photo-degradation of zinc oxide filled polypropylene nanocomposites, Polymer 47 (9) (2006) 3207–3217. [35] R. Yang, Y. Li, J. Yu, Photo-stabilization of linear low density polyethylene by inorganic nanoparticles, Polym. Degrad. Stab. 88 (2) (2005) 168–174. [36] U. Galan, H.A. Sodano, Intermolecular interactions dictating adhesion between ZnO and Graphite, Carbon 63 (2013) 517–522. [37] G.J. Ehlert, U. Galan, H.A. Sodano, Role of surface chemistry in adhesion between ZnO nanowires and carbon fibers in hybrid composites, ACS Appl. Mater. Interfaces 5 (3) (2013) 635–645.
Declaration of competing interest We declare that we do not have any commercial or associative in terest that represents a conflict of interest in connection with the work submitted. Acknowledgments Authors are grateful to the financial support by Textile Vision Basic Research Project. We thank LetPub for its linguistic assistance during the preparation of this manuscript. Appendix A. Supplementary data Supplementary data related to this article can be found at https://do i.org/10.1016/j.compscitech.2020.107996. References [1] C. Bakis, L.C. Bank, V. Brown, E. Cosenza, J. Davalos, J. Lesko, A. Machida, S. Rizkalla, T. Triantafillou, Fiber-reinforced polymer composites for constructionstate-of-the-art review, Compos. Constr. 6 (2002) 73–87. [2] M.A. Said, B. Dingwall, A. Gupta, A.M. Seyam, G. Mock, T. Theyson, Investigation of ultra violet (UV) resistance for high strength fibers, Adv. Space Res. 37 (11) (2006) 2052–2058. [3] H. Zhang, J. Zhang, J. Chen, X. Hao, S. Wang, X. Feng, Y. Guo, Effects of solar UV irradiation on the tensile properties and structure of PPTA fiber, Polym. Degrad. Stab. 91 (11) (2006) 2761–2767. [4] J. Yuan, Z. Zhang, M. Yang, W. Wang, X. Men, W. Liu, POSS grafted hybrid-fabric composites with a biomimic middle layer for simultaneously improved UV resistance and tribological properties, Compos. Sci. Technol. 168 (2018) 69–78. [5] C.Y. Yue, G.X. Sui, H.C. Looi, Effects of heat treatment on the mechanical properties of Kevlar-29 fibre, Compos. Sci. Technol. 60 (3) (2000) 421–427. [6] B.K. Little, Y. Li, V. Cammarata, R. Broughton, G. Mills, Metallization of Kevlar fibres with gold, ACS Appl. Mater. Interfaces 3 (6) (2011) 1965–1973. [7] X. Zhao, K. Hirogaki, I. Tabata, S. Okubayashi, T. Hori, A new method of producing conductive aramid fibers using supercritical carbon dioxide, Surf. Coat. Technol. 201 (2006) 628–636. [8] S.E. Atanasov, C.J. Oldham, K.A. Slusarski, J. Taggart-Scarff, S.A. Sherman, K. J. Senecal, S.F. Filocamo, Q.P. McAllister, E.D. Wetzel, G.N. Parsons, Improved cutresistance of Kevlar® using controlled interface reactions during atomic layer deposition of ultrathin (<50 Å) Inorganic Coatings, J. Mater. Chem. 2 (2014) 17371–17379. [9] L.S. Li, L.F. Allard, W.C. Bigelow, On the morphology of aromatic polyamide fibers (Kevlar, Kevlar-49, and PRD-49), J. Macromol. Sci. B 22 (2) (1983) 269–290. [10] S.C. Simmens, J.W.S. Hearle, Observation of bands in high-modulus aramid fibers by polarization microscopy, J. Polym. Sci. Phys. 18 (4) (1980) 871–876. [11] S. Li, A. Gu, G. Liang, L. Yuan, J. Xue, A facile and green preparation of poly (glycidyl methacrylate) coated aramide fibers, J. Mater. Chem. 122 (2012) 8960–8968. [12] V. Vilay, M. Mariatti, R. Mat Taib, M. Todo, Effect of fiber surface treatment and fiber loading on the properties of bagasse fiber–reinforced unsaturated polyester composites, Compos. Sci. Technol. 68 (2008) 631–638. [13] J. Lin, Effect of surface modification by bromination and metalation on Kevlar fibre-epoxy Adhesion, Eur. Polym. J. 38 (1) (2002) 79–86. [14] T. Liu, Y. Zheng, J. Hu, Surface Modification of aramid fibers with new chemical method for improving interfacial bonding strength with epoxy resin, J. Appl. Polym. Sci. 118 (5) (2010) 2541–2552.
8
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Covalent functionalization of aramid fibers with zinc oxide nano-interphase for improved UV resistance and interfacial strength in composites Lixiang Ma, Jingwei Zhang, Cuiqing Teng * State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, 201620, PR China
A B S T R A C T
Improving the resistances of organic high-performance fibers to harsh environments and enhancing the interfacial interactions of fiber-reinforced composites have become crucial in various applications. In this report, ZnO nanoparticles (NPs) and ZnO nanowires (NWs) were successfully “grown” on the surfaces of aramid fibers (AFs) by grafting with γ-aminopropyl triethoxysilane (KH550) followed by the growth of nano-ZnO. The surface functionalized AFs exhibited improved UVresistances. After 168 h of ultraviolet exposure, the tensile retention rates of the ZnO-NP- and ZnO-NW-grafted AFs reached 95.6% and 97.7%, respectively, which were significantly higher than the value of 79.1% of the bare fiber. Meanwhile, the introduction of the KH500 and ZnO formed a nano-interphase, enhancing the interfacial strength of the fiber-reinforced epoxy resin composites. The interfacial shear strengths (IFSSs) of the composites with AF-g-ZnO NPs and AF-g-ZnO NWs were 42.9 and 47.8 MPa, respectively, whereas that of bare AF-reinforced epoxy resin was only 31.2 MPa. Nano-ZnO was physically deposited on the AF surfaces without KH550. The IFSS was 36.4 MPa for the AF-ZnO NP and 38.8 MPa for the AF-ZnO NW, which were lower than those of the grafted composites. Therefore, the chemical grafting nano-ZnO on high-performance fibers provides a new strategy for improving the UV-resistances of advanced fibers and to enhance the mechanical properties of fiber-reinforced composites.
1. Introduction Aramid fibers (AFs), such as Kevlar®, Technora® or Rusar®, exhibit superior performances, including high thermal stabilities, good chemi cal resistances, and excellent specific tensile strengths. Thus, fiberreinforced polymer composites have been extensively used in various fields, including aerospace, the military, the automotive industry, and the energy industry [1]. As a high-performance fiber, the amide-bond structures of AFs are easily broken under UV light, causing aging degradation of the fiber when it is exposed to harsh environmental conditions with high temperatures and ultraviolet (UV) radiation [2–5]. To prevent the loss of mechanical properties, methods to improve the UV resistance have been evaluated, such as coating a resin or metal oxide on the fibers [6–8]. At present, the main method to improve the UV resis tance is grafting a UV absorber on the AFs. Organic UV absorbers generally have poor thermal resistances and limited lifespans, whereas inorganics can indefinitely absorb UV radiation due to their inherent structures. ZnO is an excellent UV absorber, and ZnO nanoparticles can be coated on the surfaces of fibers to improve their UV resistances in nature. However, there are not strong chemical interactions between ZnO and fibers, resulting in debonding at the interfacial phase, thereby leading to the deterioration or even failure of the composites.
It is well known that the mechanical properties of fiber-reinforced polymer composites are influenced by the interfacial characteristics between the fiber and resin matrices. However, the surfaces of the AFs are chemically inert and smooth due to the high crystallinities of the fiber surface layers and the lack of polar functional groups in the macromolecular chains, resulting in a poor adhesion between the AFs and the resin matrix [9,10]. Therefore, surface modification is essential to enhance the interfacial bonding strengths of AFs. Various approaches for surface modification have been developed to reinforce the interfacial interactions between fibers and matrices, such as chemical etching [11, 12], grafting [13–15], plasma treatment [16], γ-ray radiation [17], electrochemical oxidation [18], and various other treatments [19]. Among them, the chemical modification approaches are convenient, stable, and suitable for industrial batch processes, forming stable chemical bonds between the fiber and matrix via chemical reactions. For instance, the interfacial shear strength interfacial shear strength (IFSS) of poly(p-phenylene-benzimidazole terephthalamide) (PBIA) fiber and epoxy was improved by nearly 27% by grafting 4-(bromomethyl) ben zoic acid on the fiber [20]. One promising approach for the fabrication of hybrid composites is the growth of inorganic nanowire arrays on the fiber surface to provide improved interfacial strength and out-of-plane reinforcement. As reported by Sodano et al. in 2009 [21,22], a novel
* Corresponding author.; E-mail address: [email protected] (C. Teng). https://doi.org/10.1016/j.compscitech.2020.107996 Received 16 April 2019; Received in revised form 5 November 2019; Accepted 5 January 2020 Available online 10 January 2020 0266-3538/© 2020 Elsevier Ltd. All rights reserved.
L. Ma et al.
Composites Science and Technology 188 (2020) 107996
functionalization method for aramid or carbon fibers was developed to enhance the bonding of a ZnO nanowire interphase grown on the fiber surface for interfacial strength enhancement. Moreover, they controlled the morphologies and arrays of the nanowires, which had significant effects on various properties, including the interfacial interactions, impact properties, and interyarn friction [23–27]. As mentioned above, simultaneously overcoming the two major inherent shortcomings of AFs has attracted widespread attention in in dustry and academia. In this study, the commercial coupling agent γ-aminopropyl triethoxysilane (KH550) was used to modify the surfaces of AFs by chemical grafting, and fiber-grafted Si–OH was successfully prepared. The ZnO nanoparticles containing hydroxyl groups were subsequently grafted on the surfaces that were modified by a dehydra tion reaction, and ZnO nanowires further “grew” from the active seeds of the ZnO nanoparticles on the surfaces of the AFs. The functionalized AFs exhibited improved UV resistance and enhanced interfacial strength between the fibers and matrices. Thus, the chemical grafting of nanoZnO on high-performance fibers provides a new method for over coming the crucial issues of UV resistance and interfacial interactions.
2.2. Surface functionalization of AFs The functionalization of the aramid fiber included hydrolysis treat ment with alkali followed by acidification, surface treatment with acyl chloride and KH550, the grafting of ZnO nanoparticles, and the growth of ZnO nanowires (NWs), as illustrated in Fig. 1. The aramid fibers were washed successively in acetone and ethanol solutions to the remove surface sizing agent, after which the fiber was rinsed with deionized water and dried. A common hydrolysis process was used to cleave the peptide bonds between the amide and carbonyl functional groups that are inherent to the aramid structure [21,28]. First, the fiber was immersed in a 30 wt% NaOH aqueous solution for 30 min, after which it was washed three times with water and immersed in a 33% HCl aqueous solution for 30 s. The carboxyl functionalized aramid fibers (AF-COOH) was thereby obtained after washing with deionized water and drying in a vacuum at 120 � C for 1 h. Second, the AF-COOH fibers were further modified chemically with a solution of 100 mL of SOCl2 and 5 mL of DMF at 30 � C for 72 h to obtain acyl chloride functionalized aramid fibers (AF-COCl). The AF-COCl fibers were subsequently washed with tetrahydrofuran (THF) and immersed into a THF solution containing 5 wt% KH550 at 60 � C for 4 h. With the reaction between the –NH2 groups of the KH550 and the -COCl groups of the above functionalized AFs, Si–OH was introduced to the fiber sur faces. The surface-modified fibers (AF-KH550) were subsequently washed with THF to remove residual KH550 and dried in vacuum at 80 � C for 2 h. A zinc oxide seed solution was synthesized using the method re ported by Wong et al. [29], as follows. Two solutions of 8 mM sodium hydroxide in ethanol and 1.6 mM Zn(CH3COO)2⋅2H2O in ethanol were prepared. A zinc acetate solution was preheated to 60 � C, after which the sodium hydroxide was added with vigorous stirring. After 30 min, the seed solution was quenched in ice water to slow the nanoparticle (NP) growth. The modified aramid fibers (AF-KH550) were dipped into the seeding solution at 60 � C for 20 min and annealed at 120 � C for 20 min. This was repeated three times to ensure that the nanoparticles were grafted uniformly onto the fiber surfaces, and thus, the AF-g-ZnO NP sample was obtained. Next, the AF-g-ZnO NP sample was added to the growth solution for the ZnO nanowires at 87 � C for 5 h, which contained 25 mM Zn(NO3)2⋅6H2O, 25 mM HMTA, and 3 mM PEI in deionized water. After 5 h, the fibers were removed, washed with deionized water,
2. Experiments 2.1. Materials Benzimidazole-contained aramid fiber (AF) used in this research was supplied from Chengrand Research Institute of Chemical Industry Co. Ltd., whose chemical structure is similar to Rusa® of poly(p-phenylenebenzimidazole terephthalamide) (PBIA). The filament diameter was 17 μm, and the tensile strength of the fiber is 5.23 GPa. Analytical grade regents of zinc acetate dihydrate (Zn(CH3COO)2⋅2H2O, 99.0%), zinc nitrate hexahydrate (Zn(NO3)2⋅6H2O, 99.0%) and polyethyleneimine (PEI, Mn ¼ 600, 99.0%) were obtained from Aladdin Co., Ltd. N,Ndimethylformamide (DMF), tetrahydrofuran (THF), thionyl chloride (SOCl2, 99.0%), sodium hydroxide (NaOH, 99.0%) and hexamethy lenetetramine (HMTA, 99.9%) were supplied from Sinopharm Chemical Reagent Co., Ltd. γ-Aminopropyl triethoxysilane (KH550) was supplied by Sigma-aldrich (Shanghai) Trading Co., Ltd. Two-component resin, including epoxy resin WSR618 (E51) and phenolic aldehyde amine hardener (T31), was obtained from Sinopharm Chemical Reagent Co., Ltd.
Fig. 1. Schematic of the preparation of the AF-g-ZnO NWs. 2
L. Ma et al.
Composites Science and Technology 188 (2020) 107996
and dried at 120 � C. The fibers obtained using KH550 were denoted as AF-g-ZnO NW, and those without KH550 were denoted as AF-ZnO NW. 2.3. Characterization Attenuated total reflectance-infrared spectroscopy (ATR-IR) was performed on a Nicolet 8700 spectroscope. X-ray photoelectron spec troscopy (XPS) was carried out on an Escalab 250Xi using a mono chromatic Al X-ray source (987.9 W, 1486.6 eV), and all samples were analyzed using Avantage software for peak fitting and integration. The structures of samples were analyzed using X-ray diffraction (XRD) with D/max-2550 (Rigaku, JAN). The size distributions of the ZnO nano particles were measured using a particle size analyzer (Zetasizer Nano ZS, UK). The surface morphologies of the fibers were observed using field-emission scanning electron microscopy (SEM, Hitachi SU8010, JAN), and the morphologies of the ZnO nanoparticles were observed using transmission electron microscopy (TEM, JEM-2100F, JAN). Tapping-mode atomic force microscopy (AFM) measurements were obtained using a Dimension FastScan Bio (Bruker, USA). Ultraviolet absorption spectra of the samples were obtained using a Lambda 45 (PE, USA). The mechanical properties were measured using an XQ-1 tensile testing instrument with a gauge length of 20 mm at a strain rate of 1% s 1. For each fiber sample, at least 15 filaments were tested. UV radia tion tests were conducted by exposing the fibers to a UV light source with a wavelength range of 290–365 nm at 3 kW for 168 h and a relative humidity of 60% using an instrument manufactured by the Nanjing Wuhe Test Equipment Co. Ltd. (China). Single-fiber microdroplet tests were performed to measure the IFSSs of the fibers modified with an epoxy matrix. The fibers were embedded in an epoxy mixture of the WSR618 epoxy resin and a phenolic aldehyde amine hardener T31 with a weight ratio of 10:3. Single-fiber pull-out testing was performed on an XQ-1 tensile testing instrument, and the size of the droplet was measured using optical microscopy (Lecia DM2500P). The IFSS was calculated using the following equation [30]:
τIFSS ¼
F
π df L
Fig. 2. ATR-FTIR spectra of AF, AF-COOH, AF-COCl, and AF-g-KH550.
reaction was presumed to occur mainly on the surface, the intensity of the fiber bulk was so high that the tiny change on the surface almost was ignored. Moreover, the band around 675 cm 1 was assigned to the C–Cl stretching vibrations of acid chloride, as evident for the AF-COCl and AF-g-KH550 samples. For the KH550-grafted AF, the absorption peaks at 2937 and 2869 cm 1 were attributed to asymmetric and symmetric stretching vibrations, respectively, of methyl and methylene groups. The peaks at 1110 and 1018 cm 1 were attributed to Si–O stretching vi bration and Si–O–C stretching vibrations, respectively [31]. The signif icant enhancement of the absorption peak at 1032 cm 1, which was attributed to the Si–O–C stretching vibrations, proved that KH550 was successfully grafted to the AF surface via covalent bonds. To further investigate the surface structures of the fibers with various treatments, XPS spectra of the AFs, AF-COOH, AF-COCl, AF-g-KH550, and AF-g-ZnO were obtained, as shown in Fig. 3, and the correspond ing elements are listed in Table 1. After hydrolysis treatment, no sig nificant changes in the C and N content were evident, except for a slight increase in the O content. The presence of Cl, Si, and Zn in the AF-COCl, AF-g-KH550, and AF-g-ZnO samples indicated that the AFs were suc cessfully modified by the acid chloride, KH550, and grafted ZnO NPs. Fig. 4 shows the C1s core-level spectra of the functionalized fibers. The C1s core-level spectrum of the pristine AFs was curve-fitted with four peak components, with binding energies (BE’s) at 284.6 eV for C–C, – N, and 288.0 eV for O– – C–NH. In 285.4 eV for C–N, 286.4 eV for N–C– – O (288.9 eV) and Si–C Fig. 4(b) and (c), with the emergence of the O–C– (284.1 eV) peaks, the formation of COOH produced by amide hydrolysis and the grafting with KH550 was evident, confirming that KH550 was
(1)
where F is the tensile force at debonding, df is the diameter of the tested fiber, and L is the embedded length of the epoxy microdroplet. Twenty samples were tested, and the average value was computed. 3. Results and discussion 3.1. Graft reaction on AF surface Directly coating nano-ZnO on the fibers was difficult due to the weak adhesion between the inorganic nanowires and the organic aramid fi bers, and the coating easily fell off the surfaces of the fibers. To improve the interaction of the ZnO nanowires with the highly crystalline aramid fibers, a fiber surface functionalization was performed, as reported by Sodano et al. [21,22]. We further modified the surface functionalization using a silane coupling agent (KH550). Fig. 1 illustrates the detailed procedure for preparing functionalized aramid fibers (AF-g-ZnO), in which nano-ZnO was grafted to fiber surfaces due to the KH550. The schematic of the preparation of the AF-g-ZnO consisted of four steps: hydrolysis treatment of AFs with alkali followed by an acidification modification, surface treatment with acyl chloride, introduction of sili con–oxygen bonds due to the KH550, grafting of the ZnO nanoparticles, and the growth of ZnO nanowires. Fig. 2 shows the ATR-FTIR spectra of the untreated AFs, hydrolyzed AFs (AF-COOH), acylated AFs (AF-COCl), and KH550-grafted AFs (AF-g– O stretching peak at 1640 cm 1, KH550). For the untreated AFs, the C– 1 N–H bending peak at 1545 cm and C–N stretching peak at 1310 cm 1 were evident. Apparently, the sodium hydroxide treatment resulted in a slight decrease in the relative intensity of the C–N absorption. Since the
Fig. 3. Wide-scan XPS spectra of the AFs, AF-COOH, AF-COCl, AF-g-KH550, and AF-g-ZnO NPs. 3
L. Ma et al.
Composites Science and Technology 188 (2020) 107996
Zetasizer Nano ZS. As shown in Fig. 6(b), the average sizes of the par ticles was 12.8 � 1.45 nm, which was in good agreement with the value obtained by TEM. Particles with a narrow distribution were uniformly coated on the fiber surface, limiting the formation of defects that could weaken the interface. In addition to the uniformity of the nanoparticles, the crystal structure also affected the strength of the bonding to the fiber surface. Previous studies found that the wurtzite structure was respon sible for the strong intermolecular bonding between the zinc oxide and functional groups on fibers [32,33].
Table 1 Elemental contents of aramid and modified fibers. Sample
C
N
O
Cl
Si
Zn
AF AF-COOH AF-COCl AF-g-KH550 AF-g-ZnO
80.16 79.70 72.89 58.94 53.07
8.13 7.09 7.24 9.03 3.25
11.71 13.21 16.06 23.93 32.3
– – 3.81 0.57 –
– – – 7.53 3.12
– – – – 8.26
successfully grafted onto the AF surfaces by covalent bonds, which was consistent with the ATR-FTIR results. Fig. 5 provides the XRD patterns of the pristine AFs, AF-COOH, AFCOCl, AF-g-KH550, AF-g-ZnO, and ZnO NPs. All the fibers exhibited similar patterns at 2θ ¼ 20� , corresponding to the AF crystallinity, which demonstrated that the modification processes, including hydrolysis, acidification, and acylation, did not damage the crystal structure of the AFs. A small diffuse peak appeared at approximately 5.3� due to longrange order in the AF-g-KH550 and AF-g-ZnO. This suggested that KH550 was grafted to the AFs due to the dehydration reaction of the –OH groups of KH550 and nano-ZnO. The diffraction pattern of ZnO confirmed that the wurtzite crystal structure was present, according to the JCPDS (no. 36–1451). The AFs grafted with ZnO NPs with KH550 also showed a corresponding crystal structure. 3.2. Surface morphologies of fibers Fig. 6(a) shows TEM images of the ZnO particles grown from Zn (CH3COO)2 at 55 � C for 0.5 h. The particles were spherical approxi mately from the lattice stripes, with diameters of ~10 nm. The sizes and distribution of the particles in the colloidal solution were measured on
Fig. 5. XRD patterns of the AFs, AF-COOH, AF-COCl, AF-g-KH550, AF-g-ZnO, and ZnO NP.
Fig. 4. C1s core-level of pristine AFs, AF-COOH, AF-COCl, and AF-g-KH550. 4
L. Ma et al.
Composites Science and Technology 188 (2020) 107996
Fig. 6. (a) TEM image (b) size distribution of ZnO NPs.
The surface modification caused remarkable changes in the surface topographies of the fibers, as shown in Fig. 7. The surface of a pristine AF (Fig. 7(a)) exhibited an extremely smooth surface with tiny grooves that resulted from the spinning process, and a similar morphology was observed in the AF-COOH (Fig. 7(b)). However, the grooves on the surface of the AF were significantly deepened due to the acylation treatment (Fig. 7(c)). At a high magnification (inset), an extremely thin polymer coating with some small spots was observed on the surface of the AF-g-KH550 (Fig. 7(d)). The AF-g-ZnO NPs were evident in the top-
down images of the AF-grafted ZnO NPs (Fig. 7(e) inset). The surface of the AF was uniformly covered with nanoparticles, resulting in fewer defects and a rougher surface. Further, the growth of the ZnO NW array was more regular. As shown in Fig. 7(f), ZnO NWs grew vertically and uniformly on the AF surface, and the nanowires had an average length of 1–2 μm. AFM images further confirmed the surface topographies of the functionalized fibers. As shown in Fig. 8, the untreated AF had a few relatively neat shallow grooves with a small roughness of 2.35 nm. No
Fig. 7. SEM images of (a) untreated AF, (b) AF-COOH, (c) AF-COCl, (d) AF-g-KH550, (e) AF-g-ZnO NP, and (f) AF-g-ZnO NW. 5
L. Ma et al.
Composites Science and Technology 188 (2020) 107996
original morphology, and there was no damage on the fibers grafted with ZnO, as shown in Fig. 10(c) and (d). The nano-ZnO functionaliza tion protected the outer surface of the AFs from the surface etching caused by UV irradiation, resulting in less deterioration of the tensile strength.
changes were evident on the surface of the AF-COOH (3.42 nm) relative to the AF. Fig. 8(c) indicated that the AF-COCl surface became much rougher (7.83 nm) than that of the AF-COOH, which was in good agreement with the SEM image of the AF-COCl. The KH550-grafted AFs exhibited many spots on the surfaces of the fibers with a significantly increased roughness (16.51 nm), as shown in Fig. 8(d). When the ZnO NPs were grafted to the surfaces of the fibers, many more tiny spots appeared, corresponding to a roughness of 21.44 nm. The rougher sur face morphology could not only increase the contact area but also strengthen the mechanical interlocking between the fiber and resin matrix. To study the role of the KH550 in the interphase between the aramid fibers and ZnO NWs, a sample of the AF-ZnO NWs without KH550 was prepared, as described in the Experimental section. The fibers of the AFg-ZnO NW and AF-ZnO NW were destroyed. As shown in Fig. 9, the ZnO NW coating without KH550 peeled off the aramid fiber in the interphase region. On the contrary, KH550 modified ZnO NWs firmly adhered to the AF surface, forming a strong nano-interface. The root of the nano wires were tightly bound to the surface of the fiber, and the debonding behavior did not occur when the interface was damaged in the AF-g-ZnO NW sample. The above results showed that the ZnO grafting with KH550 did not hinder the growth of the ZnO NWs on the fiber surface but significantly improved the adhesion of the ZnO to the fibers instead.
3.4. Interfacial adhesion of fibers/epoxy composites Micro-debonding tests were carried out to assess the effect of the ZnO graft modification on the interfacial adhesion of the composites. Two methods to prepare the modified fibers were compared, i.e., physical deposition and chemical grafting methods. The AFs modified using a ZnO physical deposition method without KH550 were named AF-ZnO NP and AF-ZnO NW, and the AFs modified by ZnO chemical grafting with KH550 were named AF-g-ZnO NP and AF-g-ZnO NW. AF-reinforced epoxy composites were prepared, and the interfacial shear strength (IFSS) results are shown in Fig. 11. For the composites reinforced by the bare AF, the IFSS was 31.2 MPa, and the debonding section (Fig. 12 (b)) was very smooth, indicating that there was weak adhesion between the fibers and epoxy resin. With the physical deposition of the ZnO NPs on the AF surface, the IFSS increased by 16.7% compared to that of the bare AF composites. The further growth of the ZnO NWs resulted in a slight enhancement in the IFSS to 38.8 MPa. However, the ZnO NPs easily fell off the surfaces of the fibers, and debonding occurred between the fibers and ZnO, as shown in Fig. 12 (c) and (d). The interfacial properties have an important effect on the bonding of ZnO and fiber. As reported by Sodano et al., for the nano-ZnO/carbon fibers system, ZnO obtains a negative charge after approaching the electron rich surface of oxidized sp2 bonded carbon surface [36], and correlation between interface strength and ketone groups was the highest, due to the limited steric hindrance of the two lone pairs on the oxygen atom [37]. In this work, the chemical grafting method was used to introduce nano-ZnO onto the surfaces of AFs with KH550, and interfacial adhesion between the fiber and resin significantly improved. As shown in Fig. 11, the IFSS values of the composites with AF-g-ZnO and AF-g-ZnO NW increased to 42.9 MPa and 47.8 MPa, respectively, corresponding to 17.8% and 23.2% increases compared to AF-ZnO NP and ZnO NW samples, respectively. Meanwhile, the debonding section underwent significant changes. As shown in Fig. 12(e), the skin of the fiber was completely stripped due to strong shearing effect between the fiber and epoxy resin, and the diameter of the AF-g-ZnO NW after debonding significantly decreased, indicating a strong bonding strength between the fibers and epoxy resin. This reinforcement can be explained by several mechanisms acting on the newly integrated interphase. On the one hand, the nanoparticles or nanowires increased the surface roughness characteristics of the fibers, allowing greater surface in teractions with the wetted epoxy matrix and a small degree of me chanical interlocking. On the other hand, ZnO was grafted to the AF
3.3. UV resistances of fibers ZnO has excellent UV shielding properties, and its shielding effect is mainly achieved by UV absorption and scattering. In the near-UV region, ZnO mainly converts UV irradiation to heat by means of electron exci tation and recombination. Many studies have been conducted to prove that ZnO fillers and films have a protective effect on organics, and the poor UV resistance of AFs is mainly attributed to the amide groups in the fibers, which easily decompose under UV irradiation [34,35]. Over the span of 168 h of UV irradiation, the mechanical properties of pristine and coated AFs were compared, as shown in Table 2. The tensile strength of the pristine fiber was 5.23 GPa, while the value decreased to 4.14 GPa due to the UV irradiation, and the strength retention rate was 79.1%. For the AF-g-KH550, the tensile strength of the fiber without UV irradiation was 4.95 GPa, which was a slight decrease due to the hydrolysis treatment. The value for the UVirradiated fibers decreased to 4.19 GPa, with a strength retention rate of 84.6%. However, for the AF-g-ZnO NP and AF-g-ZnO NW samples, the strength retention rates were 95.6% and 97.7%, respectively, which are significantly different from that of the pristine AF. Apparently, the modification with nano-ZnO significantly improved the UV resistance for the high-performance fibers. The SEM image in Fig. 10(b) shows the damaged surface of the bare fibers due to the severe UV-etching and the creation of vertical cracks. The ZnO-coated fibers maintained the
Fig. 8. AFM images of the surface morphologies of (a) untreated AF, (b) AF-COOH, (c) AF-COCl, (d) AF-g-KH550, and (e) AF-g-ZnO NP. 6
L. Ma et al.
Composites Science and Technology 188 (2020) 107996
Fig. 9. SEM images of the peeled interphases of (A) AF-ZnO NWs and (B) AF-g-ZnO NWs. Table 2 Effects of UV irradiation on tensile strengths of aramid fibers. Samples F3 F3-g-KH550 F3-g-ZnO NP F3-g-ZnO NW
Tensile strength (GPa) No UV irradiation
UV irradiated
5.23 4.95 4.77 4.71
4.14 4.19 4.56 4.60
Strength retention rate (%) 79.1 84.6 95.6 97.7
Fig. 12. SEM images of micro-debonding test samples: (a) micro-debonding sample and (b–f) fracture morphologies after debonding. Fig. 10. SEM micrographs of AFs before and after UV irradiation: (a) AFs before UV irradiation, (b) AFs after UV irradiation, (c) AF-g-ZnO NP after UV irradiation, and (d) AF-g-ZnO NW after UV irradiation.
surfaces by silane coupling agents, which further strengthened the interphase between the fibers and ZnO. Overall, the improved interac tion demonstrated that the ZnO-grafted AFs had stronger interactions with the epoxy matrix compared with the ZnO-deposited AFs. 4. Conclusions Nano-ZnO was uniformly grafted to the fiber surfaces by the “bridge effect” of the silane coupling agent KH550, which significantly improved the UV resistances of the fibers. For instance, after 168 h of UV exposure, the fiber tensile strength increased from 79.1% to 95.6% (AF-g-ZnO NP) and 97.7% (AF-g-ZnO NW). The chemical graft of the ZnO endowed the aramid fibers with high roughness characteristics, which enhanced the interactions between the aramid fibers and ZnO. For the epoxy resin composites reinforced with AF-ZnO NPs or AP-ZnO NWs, the IFSSs were 42.9 and 47.8 MPa, respectively, which were significantly higher than those of the composites without the KH550-grafted AFs. Moreover, based on the SEM observations of the micro-debonding fracture sur faces, a strong interphase for the AF-g-ZnO-reinforced epoxy resin formed, and the interfacial adhesion was significantly better than that of the ZnO-deposited AFs and epoxy resin. Therefore, this report provides an effective method to improve the UV-resistances of advanced fibers
Fig. 11. Interfacial shear strength of the modified AFs and epoxy composites.
7
L. Ma et al.
Composites Science and Technology 188 (2020) 107996
and enhance the mechanical properties of fiber-reinforced composites.
[15] G.J. Ehlert, Y. Lin, H.A. Sodano, Carboxyl functionalization of carbon fibers through a grafting reaction that preserves fiber tensile strength, Carbon 49 (13) (2011) 4246–4255. [16] G.S. Sheu, S.S. Shyu, Surface properties and interfacial adhesion studies of aramid fibres modified by gas plasmas, Compos. Sci. Technol. 52 (4) (1994) 489–497. [17] Y. Zhang, Z. Jiang, Y. Huang, Q. Li, The modification of Kevlar fibers in coupling agents by γ-ray co-irradiation, Fibers Polym. 12 (8) (2011) 1014–1020. [18] M. Andideh, M. Esfandeh, Effect of surface modification of electrochemically oxidized carbon fibers by grafting hydroxyl and amine functionalized hyperbranched polyurethanes on interlaminar shear strength of epoxy composites, Carbon 123 (2017) 233–242. [19] Z. Cheng, C. Chen, J. Huang, T. Chen, Y. Liu, X. Liu, Nondestructive grafting of PEI on aramid fiber surface through the coordination of Fe (III) to enhance composite interfacial properties, Appl. Surf. Sci. 401 (2017) 323–332. [20] Y. Dai, C. Meng, Z. Cheng, L. Luo, X. Liu, Nondestructive modification of aramid fiber based on selective reaction of external cross-linker to improve interfacial shear strength and compressive strength, Compos. Part A 119 (2019) 217–224. [21] G.J. Ehlert, H.A. Sodano, Zinc Oxide Nanowire interphase for enhanced lightweight polymer fiber composites, ACS Appl. Mater. Interfaces 1 (8) (2009) 1827–1833. [22] Y. Lin, G.J. Ehlert, H.A. Sodano, Increased interface strength in carbon fiber composites through a ZnO nanowire interphase, Adv. Funct. Mater. 19 (2009) 2654– 2660. [23] U. Galan, Y. Lin, G.J. Ehlert, H.A. Sodano, Effect of nanowire morphology on interfacial strength of nanowire coated carbon fibers, Compos. Sci. Technol. 71 (2011) 946–954. [24] H.-S. Hwang, M.H. Malakooti, H.A. Sodano, Tailorable interyarn friction of ZnO nanowire-aramid fiber composite fabrics by controlling morphology of ZnO nanowires, Compos. Part A 76 (2015) 326–333. [25] M.H. Malakooti, H.-S. Hwang, H.A. Sodano, Morphology-controlled ZnO nanowire arrays for tailored hybrid composites with high damping, ACS Appl. Mater. Interfaces 7 (1) (2015) 332–339. [26] M.H. Malakooti, H.-S. Hwang, N. Goulbourne, H.A. Sodano, Role of ZnO nanowire arrays on the impact response of aramid fabrics, Compos. Part B 127 (2017) 222–231. [27] H. Hwang, H.M. Mohammad, B.A. Patterson, H.A. Sodano, Increased interyarn friction through ZnO nanowire arrays grown on aramid fabric, Compos. Sci. Technol. 107 (2015) 75–81. [28] E.G. Chatzi, S.L. Tidrick, J.L. Koenig, Characterization of the surface hydrolysis of Kevlar-49 fibers by diffuse reflectance FTIR spectroscopy, J. Polym. Sci. Phys. 26 (8) (1988) 1585–1593. [29] E.M. Wong, J.E. Bonevich, P. Searson, Growth kinetics of nanocrystalline ZnO particles from colloidal suspensions, J. Phys. Chem. B 102 (40) (1998) 7770–7775. [30] B. Miller, P. Muri, L. Rebenfeld, A microbond method for determination of the shear strength of a fiber/resin interface, Compos. Sci. Technol. 28 (1) (1987) 17–32. [31] R. Sa, Y. Yan, Z. Wei, L. Zhang, W. Wang, M. Tian, Surface modification of aramid fibers by bio-inspired poly(dopamine) and epoxy functionalized silane grafting, ACS Appl. Mater. Interfaces 6 (23) (2014) 21730–21738. [32] G.J. Ehlert, U. Galan, H.A. Sodano, Role of surface chemistry in adhesion between ZnO nanowires and carbon fibers in hybrid composites, ACS Appl. Mater. Interfaces 5 (3) (2013) 635–645. [33] B.A. Patterson, U. Galan, H.A. Sodano, Adhesive force measurement between Hopg and zinc oxide as an indicator for interfacial bonding of carbon fiber composites, ACS Appl. Mater. Interfaces 7 (28) (2015) 15380–15387. [34] H. Zhao, R.K. Li, A Study on the photo-degradation of zinc oxide filled polypropylene nanocomposites, Polymer 47 (9) (2006) 3207–3217. [35] R. Yang, Y. Li, J. Yu, Photo-stabilization of linear low density polyethylene by inorganic nanoparticles, Polym. Degrad. Stab. 88 (2) (2005) 168–174. [36] U. Galan, H.A. Sodano, Intermolecular interactions dictating adhesion between ZnO and Graphite, Carbon 63 (2013) 517–522. [37] G.J. Ehlert, U. Galan, H.A. Sodano, Role of surface chemistry in adhesion between ZnO nanowires and carbon fibers in hybrid composites, ACS Appl. Mater. Interfaces 5 (3) (2013) 635–645.
Declaration of competing interest We declare that we do not have any commercial or associative in terest that represents a conflict of interest in connection with the work submitted. Acknowledgments Authors are grateful to the financial support by Textile Vision Basic Research Project. We thank LetPub for its linguistic assistance during the preparation of this manuscript. Appendix A. Supplementary data Supplementary data related to this article can be found at https://do i.org/10.1016/j.compscitech.2020.107996. References [1] C. Bakis, L.C. Bank, V. Brown, E. Cosenza, J. Davalos, J. Lesko, A. Machida, S. Rizkalla, T. Triantafillou, Fiber-reinforced polymer composites for constructionstate-of-the-art review, Compos. Constr. 6 (2002) 73–87. [2] M.A. Said, B. Dingwall, A. Gupta, A.M. Seyam, G. Mock, T. Theyson, Investigation of ultra violet (UV) resistance for high strength fibers, Adv. Space Res. 37 (11) (2006) 2052–2058. [3] H. Zhang, J. Zhang, J. Chen, X. Hao, S. Wang, X. Feng, Y. Guo, Effects of solar UV irradiation on the tensile properties and structure of PPTA fiber, Polym. Degrad. Stab. 91 (11) (2006) 2761–2767. [4] J. Yuan, Z. Zhang, M. Yang, W. Wang, X. Men, W. Liu, POSS grafted hybrid-fabric composites with a biomimic middle layer for simultaneously improved UV resistance and tribological properties, Compos. Sci. Technol. 168 (2018) 69–78. [5] C.Y. Yue, G.X. Sui, H.C. Looi, Effects of heat treatment on the mechanical properties of Kevlar-29 fibre, Compos. Sci. Technol. 60 (3) (2000) 421–427. [6] B.K. Little, Y. Li, V. Cammarata, R. Broughton, G. Mills, Metallization of Kevlar fibres with gold, ACS Appl. Mater. Interfaces 3 (6) (2011) 1965–1973. [7] X. Zhao, K. Hirogaki, I. Tabata, S. Okubayashi, T. Hori, A new method of producing conductive aramid fibers using supercritical carbon dioxide, Surf. Coat. Technol. 201 (2006) 628–636. [8] S.E. Atanasov, C.J. Oldham, K.A. Slusarski, J. Taggart-Scarff, S.A. Sherman, K. J. Senecal, S.F. Filocamo, Q.P. McAllister, E.D. Wetzel, G.N. Parsons, Improved cutresistance of Kevlar® using controlled interface reactions during atomic layer deposition of ultrathin (<50 Å) Inorganic Coatings, J. Mater. Chem. 2 (2014) 17371–17379. [9] L.S. Li, L.F. Allard, W.C. Bigelow, On the morphology of aromatic polyamide fibers (Kevlar, Kevlar-49, and PRD-49), J. Macromol. Sci. B 22 (2) (1983) 269–290. [10] S.C. Simmens, J.W.S. Hearle, Observation of bands in high-modulus aramid fibers by polarization microscopy, J. Polym. Sci. Phys. 18 (4) (1980) 871–876. [11] S. Li, A. Gu, G. Liang, L. Yuan, J. Xue, A facile and green preparation of poly (glycidyl methacrylate) coated aramide fibers, J. Mater. Chem. 122 (2012) 8960–8968. [12] V. Vilay, M. Mariatti, R. Mat Taib, M. Todo, Effect of fiber surface treatment and fiber loading on the properties of bagasse fiber–reinforced unsaturated polyester composites, Compos. Sci. Technol. 68 (2008) 631–638. [13] J. Lin, Effect of surface modification by bromination and metalation on Kevlar fibre-epoxy Adhesion, Eur. Polym. J. 38 (1) (2002) 79–86. [14] T. Liu, Y. Zheng, J. Hu, Surface Modification of aramid fibers with new chemical method for improving interfacial bonding strength with epoxy resin, J. Appl. Polym. Sci. 118 (5) (2010) 2541–2552.
8