Chemical Modifications of Continuous Aramid Fiber for Wood Flour/High

Article Chemical Modiﬁcations of Continuous Aramid Fiber for Wood Flour/High-Density-Polyethylene Composites with Improved Interfacial Bonding

Wanyu Liu 1,†, Yue Li 2,†, Shunmin Yi 3, Limin Wang 1, Haigang Wang 1,* and Jingfa Zhang 4,*

1 Key Laboratory of Bio-Based Material Science and Technology (Ministry of Education), Northeast Forestry University, Harbin 150040, China; [email protected] (W.L.); [email protected] (L.W.) 2 Taiyuan Science Institute of Soil and Water Conservation, Taiyuan 030000, China; [email protected] 3 Guangxi Key Laboray Chemistry & Engineering Forestry Production, Guangxi University Nationalities, Nanning 530008, China; [email protected] 4 Forest Research Institute, Université du Québec en Abitibi-Témiscamingue, Rouyn-Noranda, QC J9X 5E4, Canada * Correspondence: [email protected] (H.W.); [email protected] (J.Z.); Tel.: +86-451-82191758 (H.W.) † These authors contributed equally to this work.

Abstract: To expand the use of wood plastic composites in the structural and engineering construc- tions applications, continuous aramid fiber (CAF) with nondestructive modification was incorporated as reinforcement material into wood-flour and high-density-polyethylene composites (WPC) by extrusion method with a special die. CAF was treated with dopamine (DPA), vinyl triethoxysilane (VTES), and DPA/VTES, respectively. The effects of these modifications on compatibility between CAF and WPCs and the properties of the resulting composites were explored. The results showed that compared with the original CAF, the adhesion strength of DPA and VTES combined modified CAF and WPCs increased by 143%. Meanwhile, compared with pure WPCs, CAF after modification increased the tensile strength, tensile modulus, and impact strength of the resulting composites by Citation: Liu, W.; Li, Y.; Yi, S.; Wang, 198, 92, and 283%, respectively. L.; Wang, H.; Zhang, J. Chemical Modifications of Continuous Aramid Keywords: continuous aramid fiber; chemical modification; interfacial strength; mechanical properties Fiber for Wood Flour/High-Density- Polyethylene Composites with Improved Interfacial Bonding. Polymers 2021, 13, 236. https:// 1. Introduction doi.org/10.3390/polym13020236 With the urgent concerns of global warming and depletion of petroleum raw mate-

Academic Editor: Ki-Ho Nam rials, the development of environmentally friendly construction materials has received Received: 22 December 2020 widespread attention. Wood-polymer composites (WPC) are economic and environmental- Accepted: 5 January 2021 friendly materials that have been widely utilized in industry and production (such as Published: 12 January 2021 the building and automotive industries) due to the recyclability, low costs, dimensional stability, and other advantages of these materials [1,2]. However, plastic and wood fibers Publisher’s Note: MDPI stays neu- are incompatible in WPCs due to their polarity difference, which reduces the strengths tral with regard to jurisdictional clai- and toughness of WPCs [3,4]. For this reason, compatibilizers such as maleated poly- ms in published maps and institutio- olefins [5–7], isocyanates [8], and silanes [9] are commonly added to improve interfacial nal affiliations. compatibility and partially offset the negative effect of weak interfacial bonding of the composites. However, even with the addition of these compatibilizers, the performance of the WPCs with improved interfacial is still insufficient. In particular, its low impact resistance fails to meet the simultaneous requirements of stiffness and impact resistance for Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. structural applications. Therefore, new strategies for improving the mechanical properties This article is an open access article of WPCs are needed. distributed under the terms and con- Hybridization of wood flour with artificial fibers (such as carbon, glass, and aramid ditions of the Creative Commons At- fibers) was commonly used to enhance the mechanical strength and impact resistance of tribution (CC BY) license (https:// WPCs [10]. The addition of short fibers at an appropriate content can effectively improve creativecommons.org/licenses/by/ the performance of the resulting composites, and the properties are related to the fiber 4.0/). types and content [11,12]. Hybrid waste sisal fibers and carbon and glass fiber reinforced

Polymers 2021, 13, 236. https://doi.org/10.3390/polym13020236 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 236 2 of 14

polypropylene (PP) composites showed great mechanical and tribological performance, while the properties of composites hybrid with glass fibers were better than carbon fibers for the same hybrid ratios [13]. Commonly, glass fiber has been extensively adopted to reinforced WPCs [14,15]. The strength, modulus of elasticity, and especially the hardness of WPCs increased further by hybridizing small amounts of glass fibers [15,16]. However, adding glass fiber increases the density of resulting composites and causes severe abrasion to machines during processing [13]. Compared with glass fibers, high-performance aramid fiber (AF) possesses a low density and can be used as an ideal reinforcement material in WPCs composites [17]. Adding a small amount of aramid fiber (2–5 wt%) can significantly improve the mechanical properties of WPCs when there was a good interface [18]. How- ever, the surface of aramid fibers is inert and has poor compatibility with most matrix. To improve interfacial compatibility between aramid fibers and WPCs matrix, the fiber has been modified by chemical treatment [18,19] and fiber micro fibrillation [17]. However, even with these modification strategies, the performance of short fiber reinforced WPCs composites remains limited that significantly reduces the potential application of short fiber as reinforcement in WPCs. This problem cannot be addressed by the addition of higher amounts of fibers, as the properties of the finally fiber reinforced composites start to decrease due to fiber agglomeration during processing [11]. Thus, there are limitations of short fiber reinforced composites that remain to be addressed. Replacing short fibers with continuous fiber into WPCs can solve the problem of fiber agglomeration. Moreover, continuous fiber can withstand a greater load than short fiber and is less easily broken. Compared to short fiber, the use of continuous fiber in WPCs more obviously enhances the mechanical properties [20]. However, due to the complicated preparation process, there have few literature reports on continuous fiber reinforced WPCs. Tamrakar et al. bond a reinforcing sheet of glass fiber and polypropylene (PP) onto the surface of WPC panels using a double-belt pressing method, and observed significantly improved mechanical performance [21]. However, hot-pressing is a complicated, semi- continuous processing method, limiting its application [22]. As an alternative, the extrusion manufacturing process has been applied to fabricate continuous fiber reinforced composites due to simplicity [23–26]. To do this, a specially designed die is often used to embed continuous fibers into the extruded WPCs. The performance of continuous fiber-reinforced WPCs composites is dependent on the fiber type, fiber amount, and position of fiber in composites. Whether fiber is embedded into the WPCs or applied to the surface results in different effects on properties of the final composites [27]. In Zolfaghari’s study, 6 rovings of continuous glass fiber incorporated into WPCs increased impact, tensile strength, and flexural strength 20-, 5.9-, and 2.3-fold, respectively [25]. Effects of the amount and type of the continuous fibers on tensile, flexural, and impact strength of WPCs are investigated in our previous work, the impact strength of carbon fiber reinforced WPCs composites improved by 713.4% [26]. Besides, it found that glass fiber has the best interfacial bonding among the three different fiber types (aramid, carbon, and glass fibers). Carbon and aramid continuous fibers would be pulled-out from composites when it is broken due to the weak adhesion between fiber and WPCs. In fact, the properties and life expectancy of continuous fiber-reinforced WPCs are significantly affected by the strength and stability of their interfaces in actual application. The poor interfacial strength between continuous fiber and WPCs matrix significantly reduces the reinforcing effect of continuous fibers. On the contrary, improved properties of continuous fiber-reinforced WPCs composites were observed with improving interfacial compatibility between aramid fibers and the WPCs [18], required for the use of WPCs for construction applications. Inspired by the mussel, dopamine (DPA) was found can deposit a thin polymer film on almost any substrate through oxidative and self-polymerization [28]. DPA polymer is readily constructed on most substrates without any complex chemical method, so also used as a versatile and effective surface modifier for synthetic fibers (carbon, glass, and aramid fibers) [29–31]. Additionally, the deposited poly (dopamine) layer contains many –OH Polymers 2021, 13, 236 3 of 14

and –NH groups, which can be used for further functionalization [32,33]. Modification by DPA has been proved for improving the compatibility and performance of fiber reinforced composites [29,34,35]. Meta-aramid (MPIA) fiber was first modified by dopamine and further grafted silane KH560 by Sa and the interfacial strength between modified aramid fibers with rubber matrix increased by 62.5% [32]. Grafted polydopamine on carbon fiber can introduce functionalization groups without breaking fiber strength and increased the interfacial adhesion strength and impact properties of the final composites by 78.57 and 75.12%, respectively [35]. Effects of nondestructive fiber modification on the interfacial strength between continuous aramid fiber (CAF) and WPCs and the properties of the resulting composites were explored in this study to expand the application of WPCs in the construction materials. Specifically, the experiment investigated the effect of nondestructive surface modified CAF with dopamine (DPA) and vinyltriethoxysilane (VTES) on interfacial shear strength (IFSS) and mechanical properties of continuous aramid fiber reinforced wood and HDPE composites (CAF-WPCs). X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) was used to investigate the chemical composition and morphology of the continuous aramid fiber before and after modification. The interfacial shear strength of CAF with WPCs was characterized by single fiber pull-out tests. Creep behavior of the resulting CAF-WPCs composites was also investigated.

2. Materials and Methods 2.1. Materials Continuous aramid fibers (CAF) were provided from Sovetl Textile company, Dong- guan, China. The average linear density of CAF was 1000D×3 and each 1000D fiber bundle contains 666 single fibers. Dopamine hydrochloride (DPA) and Vinyl triethoxysilane (VTES) were purchased from Macklin Biochemical company, Shanghai, China. Tris (hydroxymethyl aminomethane) was purchased from Zhanyun Chemical company, Shanghai, China. Iron (III) chloride (FeCl3), analytical reagent grade, was provided from Research Institute of Tianjin Guangfu Fine Chemical. Other chemicals (ethanol, water, and hydrochloric acid) were commercially available and used as received. HDPE (5000S) pellets were provided by Daqing Petrochemical company, China. The melt index of this material is 0.7 g·10 min−1 (190 ◦C, 2.16 kg) and the density is 0.954 g·cm−3. Poplar wood veneers were ground into wood particles of 40–80 mesh using a grinder in the Lab. Maleic anhydride grafted polyethylene (MAPE) with the graft ratio of 0.9–1.2 wt% was purchased from Nantong Rizhisheng Polymer Materials company. Stearic acid (1801) and PE wax were commercially available in China.

2.2. Surface Modification of CAF Before modification, continuous aramid fibers (CAF) were immersed in absolute ethanol for 48 h to remove the impurities on CAF surface and washed three times with deionized water and dried at 80 ◦C for use. After modification, the modified CAF was washed three times with deionized water and dried at 80 ◦C for use.

2.2.1. VTES Treatment For VTES treatment, 95% ethyl alcohol solution was used as the hydrolysis medium. VTES and 95% ethyl alcohol solution (1:50, v/v) were mixed for hydrolysis by stirring at 50 ◦C for 2 h. Next, 95% ethyl alcohol solution was added to the hydrolyzed solution until reaching a mixing ratio of VTES and 95% ethyl alcohol of 1:500 (v/v). CAF was immersed in the VTES-95% ethyl alcohol solution at room condition for 2 h (at a mass fraction ratio of VTES and CAF of 1:10).

2.2.2. DPA Treatment Modiﬁcation of the aramid ﬁbers with DPA for deposition of poly (dopamine) layers was carried out by immersion process, as described in the previous study [32]. First, Polymers 2021, 13, x FOR PEER REVIEW 4 of 14

Polymers 2021, 13, 236 2.2.2. DPA Treatment 4 of 14 Modification of the aramid fibers with DPA for deposition of poly (dopamine) layers was carried out by immersion process, as described in the previous study [32]. First, dissolvedissolve Tris Tris in indistilled distilled water water to toprepare prepare 0.05 0.05 mol·L mol−1· LTris−1 Tris-HCl-HCl buffer buffer with with adding adding HCl HClto adjustto adjust the pH the to pH 8.5. to T 8.5.hen Thendissolve dissolve dopamine dopamine in the inTris the-HCl Tris-HCl buffer buffersolution solution to a con- to a centrationconcentration of 2.0 ofg·L 2.0−1. Finally, g·L−1. Finally, immersed immersed CAF in CAFthe dopamine in the dopamine solution solutionprepared prepared above andabove react and at room react temperature at room temperature for 24 h. for 24 h.

2.2.3.2.2.3. DPA DPA and and VTES VTES Combined Combined Treatment Treatment ForFor DPA DPA and and VTES VTES modific modification,ation, CAF waswas firstfirst modified modified by by DPA DPA treatment, treatment, and and then thenexcessive excessive FeCl FeCl3 was3 was Dissolved Dissolved in VTES-95% in VTES-95% ethyl ethyl alcohol alcohol solution solution to treat to treat the dopamine the do- paminemodified modified fiber (CAFD) fiber (CAFD) by immersing by immersing the fiber the for fiber 2 h for (using 2 h (using a mass a fraction mass fraction ratio of ratio VTES ofand VTES CAF and of CAF 1:10). of 1:10). InIn this this study, study, CAF CAF modified modified with with VTES, VTES, DPA, DPA, and and DPA DPA and and VTES VTES were were named named as as CAFV,CAFV, CAFD, CAFD, and and CAFDV, CAFDV, respectively. respectively. 2.3. Preparation of CAF-Reinforced WPCs Composites 2.3. Preparation of CAF-Reinforced WPCs Composites The preparation of continuous fiber reinforced WPCs composites was described in our The preparation of continuous fiber reinforced WPCs composites was described in previous work [26,36]. Briefly, wood flour, HDPE, MAPE, and PE wax were mixed in the our previous work [26,36]. Briefly, wood flour, HDPE, MAPE, and PE wax were mixed in mixer with a certain ratio (Table1) at ambient temperature. Using a co-rotating twin-screw the mixer with a certain ratio (Table 1) at ambient temperature. Using a co-rotating extruder, the compounds were extruded and pulverized into pellets. Then, continuous twin-screw extruder, the compounds were extruded and pulverized into pellets. Then, fibers were drawn to the special die and extruded with the melt pellets together by a single continuous fibers were drawn to the special die and extruded with the melt pellets to- screw extruder (Figure1). The CAF reinforced WPCs composites were finally formed to a gether by a single screw extruder (Figure 1). The CAF reinforced WPCs composites were 4 × 50 mm2 (thickness × width) continuous plate and there were three bundles of aramid finally formed to a 4 × 50 mm2 (thickness × width) continuous plate and there were three fibers in composites, with a mass fraction of 0.1%. bundles of aramid fibers in composites, with a mass fraction of 0.1%. Table 1. Formulation of the composites for extrusion. Table 1. Formulation of the composites for extrusion. WF HDPE MAPE PE Wax Stearic Acid WF HDPE MAPE PE Wax Stearic Acid ContentContent (wt%) (wt%) 5050 44 44 4 41 11 1

FigureFigure 1. 1.SchematicSchematic diagram diagram of ofthe the processing processing and and the the shape shape of ofCAF CAF reinforced reinforced WPCs. WPCs.

2.4.2.4. Characterization Characterization 2.4.1.2.4.1. Scanning Scanning Electron Electron Microscopy Microscopy (SEM) (SEM) Frozen the CAF reinforced composites in liquid nitrogen for 5 min and breaking along Frozen the CAF reinforced composites in liquid nitrogen for 5 min and breaking the extrusion direction. Fibers and cryo-fractured surfaces were placed on the sample along the extrusion direction. Fibers and cryo-fractured surfaces were placed on the stage and sputter-coated with gold. The scanning electron microscopy (SEM, FEI company sample stage and sputter-coated with gold. The scanning electron microscopy (SEM, FEI Quanta200, Hillsboro, TX, USA) was used to observe under a 15 kV accelerating voltage. company Quanta200, Hillsboro, TX, USA) was used to observe under a 15 kV accelerat- ing2.4.2. voltage. X-ray Photoelectron Spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientiﬁc K-Alpha, Waltham, MA, USA) was used to analyze the surface element content of virgin and modiﬁed CAF. The samples were tested at 12 kV and 6 mA, the energy resolution is 0.5 eV.

2.4.3. Mechanical Strength of CAF The mechanical strength of CAF before and after modiﬁcation was measured by a universal mechanical machine (CMT5540, MTS company, Eden Prairie, MN, USA) with the Polymers 2021, 13, x FOR PEER REVIEW 5 of 14

2.4.2. X-ray Photoelectron Spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific K-Alpha, Waltham, MA, USA) was used to analyze the surface element content of virgin and modified CAF. The samples were tested at 12 kV and 6 mA, the energy resolution is 0.5 eV.

Polymers 2021, 13, 236 2.4.3. Mechanical Strength of CAF 5 of 14 The mechanical strength of CAF before and after modification was measured by a universal mechanical machine (CMT5540, MTS company, Eden Prairie, MN, USA) with 100the mm100 ·mm·minmin−1 stretching−1 stretching speed speed according according to GB/T14337-2008. to GB/T14337-2008. Each Each group group of samples of samples was testedwas tested 10 times 10 times to determine to determine the average the average values. values.

2.4.4.2.4.4. CAF and WPCs Matrix Adhesion MeasurementMeasurement TheThe singlesingle ﬁberfiber pull-outpull-out test test was was used used on on a universala universal mechanical mechanical machine machine to measureto meas- theure interfacial the interfacial adhesion adhesion of CAF of reinforced CAF reinforced WPCs composites WPCs composites as illustrated as illustrated by the schematic by the −1 diagramsschematic indiagrams Figure2 in. The Figu crossheadre 2. The crosshead speed was speed 100 mm was ·10min0 mm·minand the−1 and interface the interface shear strengthshear strength (IFSS) (IFSS)τ calculation τ calculation formula formula is as follows: is as follows: F τ = τ = πdl where F refers to the maximum force, d is the diameter of CAF, and l is the length of test where F refers to the maximum force, d is the diameter of CAF, and l is the length of test composites. Each test was repeated 10 times to determine the average values. composites. Each test was repeated 10 times to determine the average values.

FigureFigure 2.2. IllustrationIllustration ofof thethe CAFCAF reinforcedreinforced WPCsWPCs pull-outpull-out experiment.experiment.

2.4.5.2.4.5. Mechanical Tests TheThe tensile tensile and and impact impact strength strength of CAF of CAF reinforced reinforced WPCs WPCs were tested were usingtested the using univer- the saluniversal testing machine testing machine and an impact and antester impact (JC-5, tester Chengde, (JC- China)5, Chengde, according China) to ASTM according D638-03 to andASTM a Chinese D638-03 standard and a Chinese of GB/T standard 1843.1-2008, of GB/T respectively. 1843.1-2008, In the respectively. tensile test, theIn the sample tensile is dumbbell-shapedtest, the sample is with dumbbell the size-shaped of 165 ×with20 × the4 mmsize3 of(length 165 ×× 20width × 4 mm× 3thickness (length ×), width and the × − widththickness), at the and narrow the width part is at 12.7 the mm,narrow the part tensile is 12.7 speed mm, is 5the mm tensile·min speed1. In the is 5 impact mm·min test,−1. theIn the sample impact is rectangular test, the sample with theis rectangular size of 80 × with10 × the4 mm size3 (lengthof 80 × ×10width × 4 m×mthickness).3 (length × − Testwidth carried × thickness). out with Test unnotched carried Izod out impactwith unnotched tests at a speedIzod imp of 3.8act m tests·s 1 whileat a speed the impact of 3.8 energym·s−1 while was 2the J. Eachimpact group energy of samples was 2 J. was Each tested group 5 times of samples to determine was tested the average 5 times values. to determine the average values. 2.4.6. Creep Behavior Analysis 2.4.6.The Creep short-term Behavior creep Analysis properties of CAF reinforced WPCs composites were tested using a rotationalThe short rheometer-term creep (AR2000ex, properties TA ofInstruments, CAF reinforced New Castle,WPCs DE,composites USA). The were samples tested were 32 × 10 × 3 mm3 (length × width × thickness). The test was carried out at 60 ◦C using a rotational rheometer (AR2000ex, TA Instruments, New Castle, DE, USA). The with an application of 2 MPa stress, and 30 min for loading and releasing, respectively. samples were 32 × 10 × 3 mm3 (length × width × thickness). The test was carried out at 60 3. Results and Discussion The reaction mechanisms of VTES treatment, DPA treatment, and DPA and VTES treatment on CAF are described in Figure3. For VTES treatment, the silane VTES is hydrolyzed to generated silanol, which can react with the carboxyl groups on the CAF surface through esterification (Figure3a). For DPA treatment, the polydopamine polymer is deposited on the CAF surface by monomer oxidative and self-polymerization (Figure3b ). In this process, dopamine is first oxidized to dopamine-quinone, and then the product undergoes intramolecular cyclization. After a series of oxidation or rearrangement reactions, the final monomers can branch react to each other. Finally, a thin layer of PDA forms on Polymers 2021, 13, x FOR PEER REVIEW 6 of 14

Polymers 2021, 13, x FOR PEER REVIEW 6 of 14

°C with an application of 2 MPa stress, and 30 min for loading and releasing, respectively. °C with an application of 2 MPa stress, and 30 min for loading and releasing, respec- 3. Resultstively. and Discussion

The3. Results reaction and mechanisms Discussion of VTES treatment, DPA treatment, and DPA and VTES treatment on CAF are described in Figure 3. For VTES treatment, the silane VTES is hy- The reaction mechanisms of VTES treatment, DPA treatment, and DPA and VTES drolyzed to generated silanol, which can react with the carboxyl groups on the CAF treatment on CAF are described in Figure 3. For VTES treatment, the silane VTES is hy- surface through esterification (Figure 3a). For DPA treatment, the polydopamine poly- drolyzed to generated silanol, which can react with the carboxyl groups on the CAF mer is deposited on the CAF surface by monomer oxidative and self-polymerization surface through esterification (Figure 3a). For DPA treatment, the polydopamine poly- Polymers 2021, 13, 236 (Figure 3b). In this process, dopamine is first oxidized to dopamine-quinone,6 of 14 and then mer is deposited on the CAF surface by monomer oxidative and self-polymerization the product undergoes intramolecular cyclization. After a series of oxidation or rear- (Figure 3b). In this process, dopamine is first oxidized to dopamine-quinone, and then rangementthe product reactions, undergoes the final intramolecular monomers can cyclization. branch react After to aeach series other. of oxidation Finally, a or thin rear- layer of PDA forms on the aramid fiber through self-assembly polymerization and the aramid fiberrangement through self-assembly reactions, the polymerizationfinal monomers andcan cross-linkingbranch react to (Figure each 4other.). Lee Finally, a thin cross-linking (Figure 4). Lee et al. performed a single-molecule study on the substrate and et al. performedlayer a single-molecule of PDA forms study on the on aramid the substrate fiber through and adhesive self-assembly properties polymerizatio of n and adhesive properties of mussel though the reaction mechanism between the polydopa- mussel though thecross reaction-linking mechanism (Figure 4). Lee between et al. theperformed polydopamine a single- polymermolecule and study substrate on the substrate and mine polymer and substrate remains unclear. According to their research, the oxidation remains unclear.adhesive According properties to their of research, mussel the though oxidation the reaction of DPA mechanism formed high-strength between the polydopa- of DPA formed high-strength irreversible covalent bonding on the CAF surface [28]. For irreversible covalentmine polymer bonding and on substrate the CAF remains surface [unclear.28]. For According DPA and to VTES their combinedresearch, the oxidation treatment,DPA polydopamineof and DPA VTES formed depositedcombined high-strength ontreatment, CAF irreversible surface polydopamine and covalent introduced deposited bonding active on on -OH the CAF CAF groups surface surface and [28]. in- For 3+ that can reacttroduced withDPA silane activeand VTES coupling -OH combinedgroups agent that VTES.treatment, can react Further, polydopaminewith Fe silane3+ with coupling sixdeposited coordination agent on VTES. CAF sites surfaceFurther, and Fe in- with troducedsix coordination active -OH sites groups that canthat bondcan react with with the silane -NH2 coupling groups onagent the VTES. surface Further, of poly Fe 3+ that can bond with the -NH2 groups on the surface of poly (dopamine) deposited aramid 3+ fiber [37]. The(dopamine) coordinativelywith six depositedcoordination unsaturated aramid sites that fiber sites can [37]. of bond Fe The3+ providewith coordinatively the active-NH2 groups sites unsaturated to on the the poly surface sites of of Fe poly (dopamine)provide deposited(dopamine) active fiber sites depositedand to promotethe poly aramid additional(dopamine) fiber [37]. VTES deposited The grafting coordinatively fiber onto and the promote fiber unsaturated surface. additio sitesnal VTES of Fe 3+ (Figure3c).graftingprovide onto active the fiber sites surface. to the poly (Figure (dopamine) 3c). deposited fiber and promote additional VTES grafting onto the fiber surface. (Figure 3c).

Figure 3. ProceduresFigure for 3. CAFProcedures modification for CAF: (a modiﬁcation:) vinyl triethoxysilane (a) vinyl-treated, triethoxysilane-treated, (b) dopamine-treated, (b) dopamine-treated, and (c) dopamine and Figure 3. Procedures for CAF modification: (a) vinyl triethoxysilane-treated, (b) dopamine-treated, and (c) dopamine and vinyl triethoxysilane-ctreated. vinyl triethoxysilaneand ( ) dopamine-treated. and vinyl triethoxysilane-treated.

FigureFigure 4. Potential 4. Potential polymerization polymerization mechanism mechanism of dopamine of dopamine [32]. [32]. Figure 4. Potential polymerization mechanism of dopamine [32].

3.1. Surface Morphology of CAF

SEM analysis was performed to determine the detailed micromorphology of the CAF. The pristine aramid fibers displayed a smooth and uniform surface after extraction by ethanol solvent (Figure5a). After dopamine treatment, the surface morphology of the fibers became rougher due to the poly (dopamine) deposition, caused by dopamine oxidative self-polymerization (Figure5b). Similarly, the fiber surface after VTES treatment appeared rough due to the grafting of the silane coupling agent (Figure5c). Compared with individual treatment with either pure DPA or VTES, there was obviously increased chemical deposition on the surface of fiber increased for DPA and VTES combined treatment, this result indicated that the presence of dopamine effectively promoted the grafting of VTES Polymers 2021, 13, x FOR PEER REVIEW 7 of 14

3.1. Surface Morphology of CAF SEM analysis was performed to determine the detailed micromorphology of the CAF. The pristine aramid fibers displayed a smooth and uniform surface after extraction by ethanol solvent (Figure 5a). After dopamine treatment, the surface morphology of the fibers became rougher due to the poly (dopamine) deposition, caused by dopamine oxidative self-polymerization (Figure 5b). Similarly, the fiber surface after VTES treatment Polymers 2021, 13, 236 appeared rough due to the grafting of the silane coupling agent (Figure 5c). Compared7 of 14 with individual treatment with either pure DPA or VTES, there was obviously increased chemical deposition on the surface of fiber increased for DPA and VTES combined treatment, this result indicated that the presence of dopamine effectively promoted the ontografting the CAF. of VTES Fiber modiﬁcationonto the CAF. resulting Fiber modification in larger surface resulting area and in larger reactivity, surface which area may and increase the interfacial adhesion between CAF and WPCs matrix. reactivity, which may increase the interfacial adhesion between CAF and WPCs matrix.

Figure 5. SEM pictures of CAF: (a) virgin, (b) dopamine-treated, (c) vinyl triethoxysilane-treated, Figure 5. SEM pictures of CAF: (a) virgin, (b) dopamine-treated, (c) vinyl triethoxysilane-treated, and (d) dopamine and vinyl triethoxysilane-treated. and (d) dopamine and vinyl triethoxysilane-treated.

3.2.3.2. Surface Surface Chemical Chemical Composition Composition of of CAF CAF TheThe XPS XPS spectra spectra of of the the surfacesurface ofof pristinepristine and modified modified CAF CAF were were used used to to verify verify the themechanism mechanism of of su surfacerface modification modification on on CAF CAF (Figure (Figure 6).6). The The XPS XPS spectroscopy spectroscopy showed showed the 1s 1s 1s theC C, 1sN,N, and1s, and O peaks O1s peaks at 289, at 402, 289, 536 402, eV 536, respectively eV, respectively,, in both in pristine both pristine CAF and CAF modified and modifiedCAF, indicating CAF, indicating that C, thatN, and C, N, O andelements O elements are present are present in CAF in CAF[18]. [Spectroscopy18]. Spectroscopy of the ofVTES the VTES-treated-treated aramid aramid fiber fibersurface surface showed showed the characteristic the characteristic peaks peaks in thein region the region of 151– of155151–155 eV assigned eV assigned to Si2s to and Si2s 10and0–104 100–104 eV assigned eV assigned to Si2p, torespectively. Si2p, respectively. This result This indicated result indicatedthat VTES that was VTES successfully was successfully grafted on grafted the CAF, on which the CAF, is consistent which is consistentwith the SEM with results. the SEMRelative results. atomic Relative percentages atomic percentagesof O and N ofon O the and CAF N onincreased the CAF after increased DPA modification after DPA modification(Table 2), and (Table the 2O), and and C the and O N and and C andC ratios N and of the C ratios CAF ofsurface the CAF increased surface from increased 21.6 and from2.8 (virgin 21.6 and fiber) 2.8 (virginto 23.0 fiber)and 6.1 to due 23.0 to and modification. 6.1 due to modification.These increases These can be increases explained can by bethe explained introduction by the of introduction -NH2 and -OH of -NH groups2 and on -OH the groupsfiber surface on the after fiber polydopamine surface after poly- depo- dopaminesition [31]. deposition Fe element [31 ].also Fe elementappeared also on appearedfiber treated on fiber with treated the combination with the combination of DPA and ofVTES DPA anddue VTESto the due successful to the successful complexation complexation of poly (dopamine) of poly (dopamine) deposited deposited fiber and fiber VTES and VTES (Figure6d). After complexation on fiber, the coordinatively unsaturated sites of Fe3+ provided additional active sites on the modified fiber for the grafting of VTES [37]. Therefore, the relative atomic percentages of silicon and O and C ratio of fiber surface increased from 3.3 and 21.6 to 5.75 and 26.5, respectively. Overall, dopamine promoted the grafting reaction of fiber and coupling agent in the presence of Fe3+. Polymers 2021, 13, x FOR PEER REVIEW 8 of 14

(Figure 6d). After complexation on fiber, the coordinatively unsaturated sites of Fe3+ provided additional active sites on the modified fiber for the grafting of VTES [37]. Polymers 2021, 13, 236 Therefore, the relative atomic percentages of silicon and O and C ratio8 of 14of fiber surface increased from 3.3 and 21.6 to 5.75 and 26.5, respectively. Overall, dopamine promoted the grafting reaction of fiber and coupling agent in the presence of Fe3+.

Figure 6. XPS spectraFigure of 6. aramidXPS spectra continuous of aramid fiber: continuous(a) original, ﬁber:(b) vinyl (a) triethoxysilane original, (b) vinyl- treated, triethoxysilane- (c) dopamine-treated, and (d) dopamine andtreated, vinyl (triethoxysilanec) dopamine-treated,-treated. and (d) dopamine and vinyl triethoxysilane-treated.

Table 2. Surface elementalTable 2. Surface composition elemental of CAF. composition of CAF.

Atomic PercentAtomic (%) Percent (%) Atomic RatioAtomic Ratio Sample Sample C OC NO SiN FeSi Fe O/C O/C N/C N/C CAF 80.01 17.32 2.67 - - 21.6 2.8 CAF 80.01 17.32 2.67 - - 21.6 2.8 CAFD 77.44CAFD 17.80 77.44 4.76 17.80- 4.76 -- 23.0- 23.0 6.1 6.1 CAFV 75.72CAFV 16.13 75.72 4.86 16.13 3.3 4.86 3.3 21.3 21.3 6.4 6.4 CAFDV 70.99CAFDV 18.78 70.99 2.84 18.78 5.75 2.84 1.635.75 1.63 26.5 26.5 4 4

3.3. Interfacial Shear3.3. Interfacial Strength ofShear CAF Strength of CAF The interface shearThe interface strength (IFSS)shear strength between (IFSS) CAF andbetween WPCs CAF was and characterized WPCs was characterized using using a single-fiber pull-outa single test,-fiber and pull the-out results test, and are the shown results in Figureare shown7. The in IFSSFigure value 7. The of theIFSS value of the prepared CAFD-WPCsprepared was CAFD 0.74-WPCs MPa, was 4.2% 0.74 higher MPa, than 4.2% that higher of the than CAF-WPC that of the composite CAF-WPC composite (0.71 MPa). This(0.71 increased MPa). This IFSS increased can be attributed IFSS can to be the attributed introduction to the of introduction polar groups of onpolar groups on the polydopaminethe polydopamine deposited CAF. dep Theosited -NH CAF.2 and The -OH -NH groups2 and confer -OH groups high affinity confer to high the affinity to the hydroxyl groupshydroxyl of matrixes. groups However, of matrixes. the slightHowever, improvement the slight suggestsimprovement that thesuggests number that the number of hydrogen bondsof hydrogen fiber exposed bonds is fiber limited, exposed so does is limited, not sufficiently so does contribute not sufficiently to interface contribute to inter- bonding in the composites.face bonding Treatment in the composites. with VTES Treatment alone increased with VTES the IFSS alone of the increased resulting the IFSS of the composite fromresulting 0.71 to 0.99 composite MPa, 39% from higher 0.71 to than 0.99 thatMPa, of 39% the untreatedhigher than system. that of Thethe untreated use system. of VTES improvedThe theuse surfaceof VTES polarity improved of the the inertsurface CAF, polarity which of helps the inert to improve CAF, which interface helps to improve compatibility ininterface composites. comp Usingatibility both in composites. polydopamine Using and both VTES polydopamine modification and on VTES fiber modification increased the interfacial strength (IFSS) of the resulting composites significantly. The tested CAFDV-WPC composite exhibited the highest IFSS value of 1.7 MPa, 139.4% higher than 3+ that of pristine fiber. In the presence of Fe , poly (dopamine) promoted the grafting of more coupling agents onto the fiber surface, resulting in greatly improved interfacial compatibility between phases. In this system, the polar hydroxyl and amino groups on CAF can extensively form hydrogen bonds with the wood flour. Additionally, the vinyl groups of coupling agent VTES can generate free radicals and react with the HDPE backbone Polymers 2021, 13, x FOR PEER REVIEW 9 of 14

on fiber increased the interfacial strength (IFSS) of the resulting composites significantly. The tested CAFDV-WPC composite exhibited the highest IFSS value of 1.7 MPa, 139.4% higher than that of pristine fiber. In the presence of Fe3+, poly (dopamine) promoted the grafting of more coupling agents onto the fiber surface, resulting in greatly improved interfacial compatibility between phases. In this system, the polar hydroxyl and amino groups on CAF can extensively form hydrogen bonds with the wood flour. Additionally, the vinyl groups of coupling agent VTES can generate free radicals and react with the HDPE backbone during the extrusion process [18]. As a result, a good interface can be Polymers 2021, 13, 236 formed between CAF and the WPCs in the final composites. 9 of 14 The tensile strength of virgin continuous fiber was measured as 1.7 N·dtex−1, and the strength of fiber was basically unchanged after modification (Figure 7), which indicating this modification has no negative effect on fiber strength. This is because both dopamine duringdeposition the and extrusion VTES grafting process reaction [18]. As only a result, occur a on good the interfaceCAF surface, can and be formed does not between break CAF andthe molecular the WPCs chain in the structure ﬁnal composites. of continuous aramid fiber.

FigureFigure 7. MechanicalMechanical strength strength and and IFSS IFSS of the of theCAF. CAF.

3.4. FractureThe tensile Morphology strength Analysis of virgin of Composites continuous fiber was measured as 1.7 N·dtex−1, and the strengthStructural of fiber morphology was basically characteristics unchanged of after the prepared modification continuous (Figure aramid7), which fiber indicating re- thisinforced modification WPCs were has investigated no negative to effect assess on the fiber interfacial strength. bonding This is becausebetween both CAF dopamine and depositionWPCs. For andvirgin VTES CAF graftingreinforced reaction WPCs, onlythe fracture occur on surface the CAF appeared surface, smooth and doeswith notal- break themost molecular no matrix chain adhesion, structure indicating of continuous weak interfacial aramid bonding fiber. between CAF and WPCs (Figure 8a). Compared to materials modified by either dopamine or VTES, adherents 3.4.were Fracture observed Morphology on the surface Analysis of the of fiber, Composites and the wire drawing phenomenon existed in the fracture composites, indicating that the interface was improved. Stronger interface Structural morphology characteristics of the prepared continuous aramid fiber rein- adhesion and more obvious drawing phenomenon were observed in composites rein- forcedforced WPCsusing DPA were and investigated VTES modified to assess fibers the (Figure interfacial 8d). The bonding results betweenindicated CAFincreased and WPCs. Forinterface virgin compatibility CAF reinforced between WPCs, the modified the fracture CAF surface and WPCs, appeared increasing smooth interfacial with almostad- no matrixhesion strength adhesion, (Figure indicating 7). weak interfacial bonding between CAF and WPCs (Figure8a). Compared to materials modified by either dopamine or VTES, adherents were observed on the surface of the fiber, and the wire drawing phenomenon existed in the fracture composites, indicating that the interface was improved. Stronger interface adhesion and more obvious drawing phenomenon were observed in composites reinforced using DPA and Polymers 2021, 13, x FOR PEER REVIEW 10 of 14 VTES modified fibers (Figure8d). The results indicated increased interface compatibility between the modified CAF and WPCs, increasing interfacial adhesion strength (Figure7).

FigureFigure 8.8. FractureFracture morphology morphology of ofthe the CAF CAF reinforced reinforced WPCs: WPCs: (a) virgin, (a) virgin, (b) dopamine (b) dopamine-treated,-treated, (c) vinyl triethoxysilane-treated, and (d) dopamine and vinyl triethoxysilane-treated. (c) vinyl triethoxysilane-treated, and (d) dopamine and vinyl triethoxysilane-treated. 3.5. Mechanical Properties of Composites Adding CAF significantly improved the mechanical performance of the resulting composites. Compared with pure WPCs, the tensile strength and modulus of CAF-reinforced WPCs increased by 130% and 68%, respectively (Figure 9). The tensile stress loaded on the composites can be transferred from the WPCs to CAF through interfacial shear. The continuous aramid fibers withstand most of the stress, resulting in the enhancement in strength and modulus. As the load increases, the cracks were generated and propagated in the WPCs matrix and the interface between CAF and WPCs, which ultimately destroyed the composites [26]. In this system, when the external force exceeds the interface bonding strength between CAF and WPCs matrix, the fiber pulls out of the matrix. At this time, the high-strength and modulus CAF subjected less stress than their own tensile strength. Therefore, the improved interfacial property is a major contributor to the overall strength of the composites. After modification, treated fiber improved tensile strength and modulus to a greater extent than virgin fiber because of the higher stress transfer efficiency between CAF and WPCs. Dopamine treated CAF introduced -NH2 and -OH groups on the fiber surface with high affinity for the hydroxyl groups of the matrix. The use of VTES improves the surface polarity of the inert CAF. However, the number of polar groups on CAFD and CAFV is limited, therefore insufficient to promote interfacial adhesion in the composite and resulting in a slight enhancement in tensile strength. For DPA and VTES combined treatment (CAFDV), polydopamine deposited on CAF surface and introduced active -OH groups that can react with silane coupling agent VTES. Fur- ther, in the presence of Fe3+, the -NH2 groups on the CAF can be complexed with VTES, thereby promote additional VTES grafting onto the fiber surface. Strong interfacial combination between CAFDV and WPC matrix significantly increased tensile strength of the resulting composites. The tensile strength and modulus of CAFDV-WPCs were 64.2 MPa and 2.3 GPa, which are 29 and 10% higher than that of untreated fiber (49.6 MPa and 2.1 GPa). Combined with the analysis of IFSS results, modification resulted in improved interface bonding, allowing CAF to withstand greater loads through interfacial shear.

Polymers 2021, 13, 236 10 of 14

3.5. Mechanical Properties of Composites Adding CAF significantly improved the mechanical performance of the resulting composites. Compared with pure WPCs, the tensile strength and modulus of CAF-reinforced WPCs increased by 130% and 68%, respectively (Figure9). The tensile stress loaded on the composites can be transferred from the WPCs to CAF through interfacial shear. The continuous aramid fibers withstand most of the stress, resulting in the enhancement in strength and modulus. As the load increases, the cracks were generated and propagated in the WPCs matrix and the interface between CAF and WPCs, which ultimately destroyed the composites [26]. In this system, when the external force exceeds the interface bonding strength between CAF and WPCs matrix, the fiber pulls out of the matrix. At this time, the high-strength and modulus CAF subjected less stress than their own tensile strength. Therefore, the improved interfacial property is a major contributor to the overall strength of the composites. After modification, treated fiber improved tensile strength and modulus to a greater extent than virgin fiber because of the higher stress transfer efficiency between CAF and WPCs. Dopamine treated CAF introduced -NH2 and -OH groups on the fiber surface with high affinity for the hydroxyl groups of the matrix. The use of VTES improves the surface polarity of the inert CAF. However, the number of polar groups on CAFD and CAFV is limited, therefore insufficient to promote interfacial adhesion in the composite and resulting in a slight enhancement in tensile strength. For DPA and VTES combined treatment (CAFDV), polydopamine deposited on CAF surface and introduced active -OH groups that can react with silane coupling agent VTES. Further, in the presence of Fe3+, the -NH2 groups on the CAF can be complexed with VTES, thereby promote additional VTES grafting onto the fiber surface. Strong interfacial combination between CAFDV and WPC matrix significantly increased tensile strength of the resulting composites. The tensile strength and modulus of CAFDV-WPCs were 64.2 MPa and 2.3 GPa, which are 29 and 10% higher than that of untreated fiber (49.6 MPa and 2.1 GPa). Combined with the analysis Polymers 2021, 13, x FOR PEER REVIEW 11 of 14 of IFSS results, modification resulted in improved interface bonding, allowing CAF to withstand greater loads through interfacial shear.

FigureFigure 9.9.Tensile Tensile properties properties of of the the CAF CAF reinforced reinforced WPCs. WPCs.

ComparedCompared withwith purepure WPCs,WPCs, thethe impactimpact strengthstrength ofof CAFCAF reinforcedreinforced WPCsWPCs waswas in-in- creasedcreased byby 283%283% maximally,maximally, whichwhich isis difﬁcultdifficult totoachieve achieve withwith commoncommon reinforcementreinforcement (Figure(Figure 10 10).). Compared Compared with with virgin virgin CAF, CAF, DPA DPA and and VTES-treated VTES-treated CAF CAF further further increased increased the − − impactthe impact strength strength of CAF of CAF reinforced reinforced WPCs WPCs composites composites from 21.1from kJ 21.1·m kJ·m2 to 28.0−2 to kJ28.0·m kJ·m2, an−2, increasean increase of 32.7%. of 32.7%. This This is due is todue the to enhanced the enhanced interface interface compatibility. compatibility. Improved Improved interfacial in- bondingterfacial allowedbonding theallowed impact the loading impact transferredloading transferred steadily steadily from the from WPCs the matrixWPCs tomatrix the ﬁbers,to the whichfibers, reducedwhich reduced crackgeneration crack generation and propagation and propagation at the interfaceat the interface between betw CAFeen andCAF WPCs and WPCs or in the or matrix.in the matrix. This process This process absorbed absorbed most of themost impact of the energy, impact resulting energy, inre- improvedsulting in impact improved strength. impact Besides, strength. the Besides, enhancement the enhancement of CAF on the of CAF impact on strength the impact of CAF-reinforcedstrength of CAF composites-reinforced iscomposites more obvious is more than obvious tensile strength.than tensile This strength. is because This unlike is because unlike the tensile stress, the direction of impact stress is perpendicular to the fiber direction in composites. The CAF restricts the movement of HDPE molecular chains, thus the stress dissipation in composites is confined [26]. This allowed the composites to withstand greater loads, finally resulting in improved impact strength. Interestingly, the effect of fiber modification on the interfacial shear strength of the composites is more obvious than that of mechanical properties. These results may be caused by the use of different test samples in IFSS and mechanical tests. There was a larger fiber volume fraction in the mechanical test sample (three bundles) than that in the IFSS sample (one bundle) and this may affect the formation of interface defects in composites. Cracks at the interface promote the debonding of CAF from the WPCs composites under external force. Another difference between the samples is that the samples used in the IFSS test were prepared by hot pressing method. Compared to the extrusion process, this method included an increased matrix melting time and additional pressure during the composite preparation process to fully impregnate the continuous fiber into the WPCs matrix.

Polymers 2021, 13, 236 11 of 14

the tensile stress, the direction of impact stress is perpendicular to the fiber direction in composites. The CAF restricts the movement of HDPE molecular chains, thus the stress Polymers 2021, 13, x FOR PEER REVIEW 12 of 14 dissipation in composites is confined [26]. This allowed the composites to withstand greater loads, finally resulting in improved impact strength.

Figure 10. Impact strength of the CAF reinforced WPCs. Figure 10. Impact strength of the CAF reinforced WPCs. Interestingly, the effect of fiber modification on the interfacial shear strength of the 3.composites6. Creep Property is more of obvious Composites than that of mechanical properties. These results may be caused by theThe use short of different-term creep test samplesperformance in IFSS of andcomposites mechanical was tests.usually There used was to acharacterize larger fiber thevolume creep fraction resistance in the and mechanical the interface test sample compatibility (three bundles) of the than composites that in the [5]. IFSS The sample creep mechanism(one bundle) at and molecular this may level affect for the unmodified formation of and interface modified defects CAF in composites. reinforced CracksWPCs atis mainlythe interface due to promote the movement the debonding and deformation of CAF from of the the HDPE WPCs molecular composites chain under and external the de- formationforce. Another of the difference material caused between by the the samples slippage is of that the the polymer samples molecular used in thechain IFSS at testthe interface.were prepared The addition by hot pressing of aramid method. fiber effectively Compared improved to the extrusion the creep process, resistance this method of the compositeincluded an (Figure increased 11). matrixCAF reduced melting timethe creep and additional deformation pressure of composite during thematerials composite re- gardlesspreparation of with process modification to fully impregnate or not. This the is continuous due to the fiber high into dimensionally the WPCs matrix. stable CAF restricted the movement of PE molecular chains. Besides, compared to non-modified fiber,3.6. treated Creep Property fiber-reinforced of Composites WF and HDPE composites exhibited smaller creep due to the enhancedThe short-term interfacial creepbonding performance between CAF of composites and WPCs. was The usually improved used interface to characterize provided the acreep stronger resistance interfacial and the shear interface effect compatibilityfor restraining of the the motion composites of HDPE [5]. The polymer creep mechanism molecules atat the molecular interface. level Composites for unmodified reinforced and with modified DPA CAFand VTES reinforced treated WPCs fiber isexhibited mainly duethe highestto the movement creep resistance and deformation among all fiberof the-reinforced HDPE molecular WPC composites, chain and indicating the deformation better interfacialof the material compatibility causedby in composites. the slippage of the polymer molecular chain at the interface. The addition of aramid fiber effectively improved the creep resistance of the composite (Figure 11). CAF reduced the creep deformation of composite materials regardless of with modification or not. This is due to the high dimensionally stable CAF restricted the movement of PE molecular chains. Besides, compared to non-modified fiber, treated fiber-reinforced WF and HDPE composites exhibited smaller creep due to the enhanced interfacial bonding between CAF and WPCs. The improved interface provided a stronger interfacial shear effect for restraining the motion of HDPE polymer molecules at the interface. Composites reinforced with DPA and VTES treated fiber exhibited the highest creep resistance among all fiber-reinforced WPC composites, indicating better interfacial compatibility in composites.

Figure 11. Creep curves of the CAF-reinforced WPCs.

4. Conclusions This study provided an effective and nondestructive functionalization method for continuous aramid fiber to develop high-performance continuous fiber-reinforced WPCs. Dopamine and functional silane VTES were successfully deposited and grafted on the

Polymers 2021, 13, x FOR PEER REVIEW 12 of 14

Figure 10. Impact strength of the CAF reinforced WPCs.

3.6. Creep Property of Composites The short-term creep performance of composites was usually used to characterize the creep resistance and the interface compatibility of the composites [5]. The creep mechanism at molecular level for unmodified and modified CAF reinforced WPCs is mainly due to the movement and deformation of the HDPE molecular chain and the deformation of the material caused by the slippage of the polymer molecular chain at the interface. The addition of aramid fiber effectively improved the creep resistance of the composite (Figure 11). CAF reduced the creep deformation of composite materials regardless of with modification or not. This is due to the high dimensionally stable CAF restricted the movement of PE molecular chains. Besides, compared to non-modified fiber, treated fiber-reinforced WF and HDPE composites exhibited smaller creep due to the enhanced interfacial bonding between CAF and WPCs. The improved interface provided a stronger interfacial shear effect for restraining the motion of HDPE polymer molecules Polymers 2021, 13, 236 at the interface. Composites reinforced with DPA and VTES treated fiber exhibited12 of the 14 highest creep resistance among all fiber-reinforced WPC composites, indicating better interfacial compatibility in composites.

FigureFigure 11.11. CreepCreep curvescurves ofof thethe CAF-reinforcedCAF-reinforcedWPCs. WPCs.

4.4. ConclusionsConclusions ThisThis studystudy providedprovided anan effectiveeffective andand nondestructivenondestructive functionalizationfunctionalization methodmethod forfor continuouscontinuous aramidaramid fiberfiber toto developdevelop high-performancehigh-performance continuouscontinuous fiber-reinforcedfiber-reinforced WPCs.WPCs. DopamineDopamine and and functional functional silane silane VTES VTES were wer successfullye successfully deposited deposited and graftedand grafted on the on fiber the surface as evidenced by XPS and SEM analysis. The interfacial adhesion between CAF and WPCs matrix was improved and the adhesion strength (IFSS) for materials prepared with modified CAF increased by 143%. Additionally, the tensile strength, tensile modulus, and impact strength of the modified CAF reinforced WPCs were further enhanced due to stronger interface shear force in the composites. The results indicate that suitable interfacial adhesion between components is the prerequisite to fully utilize the reinforcing ability of CAF, and strong interface bonding is critical for composites with improved properties.

Author Contributions: Investigation, Y.L., W.L. and L.W.; Data curation, W.L. and S.Y.; writing—original draft preparation, W.L.; writing—review and editing, S.Y., J.Z. and H.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Key Research and Development Program of China, grant number 2019YFD1101203, the Fundamental Research Funds for the Central Universities of China, grant number 2572020DR13, and the Research and Development Program in Key Areas of Guangdong Province, China, grant number 2020B0202010008. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: All data included in this study are available upon request by contact with the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Moghadam, S.K.; Shamsian, M.; Shahri, H.R. Manufacturing wood-plastic composites from waste lignocellulose plus haloxylon species and recycled plastics. For. Prod. J. 2019, 69, 205–209. 2. Chan, C.M.; Vandi, L.J.; Pratt, S.; Halley, P.; Richardson, D.; Werker, A.; Laycock, B. Composites of wood and biodegradable thermoplastics: A review. Polym. Rev. 2018, 58, 444–494. [CrossRef] 3. Zhou, Y.; Fan, M.; Chen, L. Interface and bonding mechanisms of plant ﬁbre composites: An overview. Compos. Part B Eng. 2016, 101, 31–45. [CrossRef] 4. Effah, B.; Raatz, K.; Reenen, A.V.; Meincken, M. Chemical force microscopy analysis of wood-plastic composites produced from different wood species and compatibilizers. Wood Fiber Sci. 2017, 49, 146–157. 5. Gao, H.; Xie, Y.; Ou, R.; Wang, Q. Grafting effects of polypropylene/polyethylene blends with maleic anhydride on the properties of the resulting wood-plastic composites. Compos. Part A Appl. Sci. Manuf. 2012, 43, 150–157. [CrossRef] Polymers 2021, 13, 236 13 of 14

6. Yi, S.; Xu, S.; Li, Y.; Gan, W.; Yi, X.; Liu, W.; Wang, Q.; Wang, H.; Ou, R. Synergistic toughening effects of grafting modification and elastomer-olefin block copolymer addition on the fracture resistance of wood particle polypropylene elastomer composites. Mater. Des. 2019, 181, 107918. [CrossRef] 7. Zhang, W.P.; Lu, Y.H.; Khanal, S.; Xu, S.A. Effects of compatibilizers on selected properties of HDPE composites highly filled with bamboo flour. Wood Fiber Sci. 2018, 50, 1–11. [CrossRef] 8. Guo, C.; Li, L.; Li, H. Evaluation of interfacial compatibility in wood flour/polypropylene composites by grafting isocyanate silane coupling agent on polypropylene. J. Adhes. Sci. Technol. 2019, 33, 468–478. [CrossRef] 9. Xie, Y.; Hill, C.S.; Xiao, Z.; Militz, H.; Mai, C. Silane coupling agents used for natural fiber/polymer composites: A review. Compos. Part A Appl. Sci. Manuf. 2010, 41, 806–819. [CrossRef] 10. Sathishkumar, T.; Naveen, J.; Satheeshkumar, S. Hybrid fiber reinforced polymer composites-a review. J. Reinf. Plast. Comp. 2014, 33, 454–471. [CrossRef] 11. Wang, H.; Zhang, J.; Wang, W.; Wang, Q. Research of fiber reinforced wood-plastic composites: A review. Sci. Silvae Sin. 2016, 52, 130–139. 12. Turku, I.; Kaerki, T. The effect of carbon fibers, glass fibers and nanoclay on wood flour-polypropylene composite properties. Eur. J. Wood Prod. 2014, 72, 73–79. [CrossRef] 13. Aslana, M.; Tufanb, M.; Küçükömero˘glu,T. Tribological and mechanical performance of sisal-filled waste carbon and glass fibre hybrid composites. Compos. Part B Eng. 2018, 140, 241–249. [CrossRef] 14. Dickson, A.R.; Even, D.; Warnes, J.M.; Fernyhough, A. The effect of reprocessing on the mechanical properties of polypropylene reinforced with wood pulp, flax or glass fibre. Compos. Part A Appl. Sci. Manuf. 2014, 61, 258–267. [CrossRef] 15. Jiang, J.; Mei, C.; Pan, M.; Lu, F. Effects of hybridization and interface modification on mechanical properties of wood flour/polymer composites reinforced by glass fibers. Polym. Compos. 2019, 40, 3602–3610. [CrossRef] 16. Almaadeed, M.A.; Kahraman, R.; Khanam, P.N.; Madi, N. Date palm wood flour/glass fibre reinforced hybrid composites of recycled polypropylene: Mechanical and thermal properties. Mater. Des. 2012, 42, 289–294. [CrossRef] 17. Zhong, L.; Fu, S.; Zhou, X.; Zhan, H. Effect of surface microfibrillation of sisal fibre on the mechanical properties of sisal/aramid fibre hybrid composites. Compos. Part A Appl. Sci. Manuf. 2011, 42, 244–252. [CrossRef] 18. Ou, R.; Zhao, H.; Sui, S.; Song, Y.; Wang, Q. Reinforcing effects of Kevlar fiber on the mechanical properties of wood-flour/high- density-polyethylene composites. Compos. Part A Appl. Sci. Manuf. 2010, 41, 1272–1278. [CrossRef] 19. Yuan, F.; Ou, R.; Xie, Y.; Wang, Q. Reinforcing effects of modified Kevlar fiber on the mechanical properties of wood- flour/polypropylene composites. J. For. Res. 2013, 24, 149–153. [CrossRef] 20. Zolfaghari, A.; Behravesh, A.H.; Shahi, P. Comparison of mechanical properties of wood-plastic composites reinforced with continuous and noncontinuous glass fibers. J. Thermoplast. Compos. 2015, 28, 791–805. [CrossRef] 21. Tamrakar, S.; Shaler, S.M.; Lopez-Anid, R.A.; Edgar, R. Mechanical property characterization of fiber-reinforced polymer wood- polypropylene composite panels manufactured using a double belt pressing technology. J. Mater. Civ. Eng. 2012, 24, 1193–1200. [CrossRef] 22. Jiang, L.; Wolcott, M.P.; Zhang, J.; Englund, K. Flexural properties of surface reinforced wood/plastic deck board. Polym. Eng. Sci. 2007, 47, 281–288. [CrossRef] 23. Kargar, M.; Behravesh, A.H.; Taheri, H.M. Experimental investigation on mechanical properties of extruded foamed PVC-wood composites reinforced with continuous glass fibers. Polym. Compos. 2016, 37, 1674–1680. [CrossRef] 24. Zolfaghari, A.; Behravesh, A.H.; Adli, A.; Sarabi, M.T. Continuous glass fiber reinforced wood plastic composite in extrusion process: Feasibility and processing. J. Reinf. Plast. Comp. 2013, 32, 52–60. [CrossRef] 25. Zolfaghari, A.; Behravesh, A.H.; Adili, A. Continuous glass fiber reinforced wood plastic composite in extrusion process: Mechanical properties. Mater. Des. 2013, 51, 701–708. [CrossRef] 26. Zhang, J.; Li, Y.; Xing, D.; Wang, Q.; Wang, H.; Koubaa, A. Reinforcement of continuous fibers for extruded wood-flour/HDPE composites: Effects of fiber type and amount. Constr. Build. Mater. 2019, 228, 116718. [CrossRef] 27. Zhou, C.; Du, F.; Wang, W. Effect of Position of Carbon Fiber Cloth on Properties of Wood Fiber/HDPE Composite. J. For. Univ. 2015, 43, 91–94. 28. Lee, H.; Dellatore, S.M.; Miller, W.M.; Messersmith, P.B. Mussel-inspired surface chemistry for multifunctional coatings. Science 2007, 318, 426–430. [CrossRef] 29. Gu, W.; Liu, X.; Ye, Q.; Gao, Q.; Gong, S.; Li, J.; Shi, S.Q. Bio-inspired co-deposition strategy of aramid fibers to improve performance of soy protein isolate-based adhesive. Ind. Crops Prod. 2020, 150, 112424. [CrossRef] 30. Cheng, K.; Li, M.; Zhang, S.; He, M.; Yu, J.; Feng, Y.; Lu, S. Study on the structure and properties of functionalized fibers with dopamine. Colloids Surf. A 2019, 582, 123846. [CrossRef] 31. Lee, W.; Lee, J.U.; Byun, J. Catecholamine polymers as surface modifiers for enhancing interfacial strength of fiber-reinforced composites. Compos. Sci. Technol. 2015, 110, 53–61. [CrossRef] 32. Sa, R.; Yan, Y.; Wei, Z.; Zhang, L.; Wang, W.; Tian, M. Surface modification of aramid fibers by bio-inspired poly(dopamine) and epoxy functionalized silane grafting. ACS Appl. Mater. Int. 2014, 6, 21730–21738. [CrossRef][PubMed] 33. Zeng, L.; Liu, X.; Chen, X.; Soutis, C. Surface modification of aramid fibres with graphene oxide for interface improvement in composites. Appl. Compos. Mater. 2018, 25, 843–852. [CrossRef] Polymers 2021, 13, 236 14 of 14

34. Hu, X.; Chen, Z.; Cao, Y.; Chen, Z.; Zhang, S.; Song, W. The effect of modifier on properties of bamboo powder/high-density polyethylene composites. For. Prod. J. 2019, 69, 313–321. 35. Gao, B.; Du, W.; Hao, Z.; Zhou, H.; Zou, D.; Zhang, R. Bioinspired modification via green synthesis of mussel-inspired nanoparticles on carbon fiber surface for advanced composite materials. ACS Sustain. Chem. Eng. 2019, 7, 5638–5648. [CrossRef] 36. LI, Y.; Zhang, J.; Yi, S.; Xiao, Z.; Wang, H. Mechanical properties of modified aramid fiber reinforced wood flour/high density polyethylene composites. Acta Mater. Compos. Sin. 2019, 36, 638–645. 37. Cheng, Z.; Chen, C.; Huang, J.; Chen, T.; Liu, Y.; Liu, X. Nondestructive grafting of PEI on aramid fiber surface through the coordination of Fe(III) to enhance composite interfacial properties. Appl. Surf. Sci. 2017, 401, 323–332. [CrossRef]