Evaluation of fiber surface modification via air plasma on the interfacial behavior of glass fiber reinforced laminated veneer lumber composites

Construction and Building Materials 233 (2020) 117315

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Evaluation of fiber surface modification via air plasma on the interfacial behavior of glass fiber reinforced laminated veneer lumber composites Wei Zhang a,b, Pei Yang a,b, Yizhong Cao a,b, Peijing Yu a,b, Minzhi Chen a,b,⇑, Xiaoyan Zhou a,b,⇑ a b

College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China Fast-growing Tree & Agro-fiber Materials Engineering Center, Nanjing 210037, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 The massive active species can be

generated from air excited by plasma.  Air plasma has the synergetic effect of

oxidation and etching.  The surface wettability of plasma

modified fiber increased significantly.  The interfacial adhesion increased

between plasma modified fiber and adhesive.  The MOR and MOE values of the plasma modified fiber reinforced LVL increased.

a r t i c l e

i n f o

Article history: Received 17 April 2019 Received in revised form 14 July 2019 Accepted 17 October 2019

Keywords: Air plasma Reinforcement Glass fiber Laminated veneer lumber Interfacial adhesion

a b s t r a c t Air plasma was successfully applied to improve the interfacial adhesion between glass fiber (GF) and adhesive for preparing the GF reinforced laminated veneer lumber (LVL) composites with excellent mechanical performance. Benefiting from the synergetic effect of oxidation and etching of air plasma, the surface free energy increased by 17.68% and water contact angle decreased by 35.9% for the plasma modified GF at 4.5 kW plasma power. The MOR and MOE values of plasma-modified composites increased by 36% and 16%, respectively. This study suggested that air plasma could become a promising technology in improving interfacial adhesion of GF reinforced LVL composites. Ó 2019 Published by Elsevier Ltd.

1. Introduction In recent decades, laminated veneer lumber (LVL), as one of the major structural composite lumbers (SCLs), has attracted increasing attention on the basis of their excellent engineering properties in a variety of structural application [1]. Even though these engineered wood products are better than traditional solid wood in terms of fire resistance, strength, stiffness and variability, whereas ⇑ Corresponding authors at: College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China. E-mail addresses: [email protected] (M. Chen), [email protected] (X. Zhou). https://doi.org/10.1016/j.conbuildmat.2019.117315 0950-0618/Ó 2019 Published by Elsevier Ltd.

the further improvements are expected to broaden the application in the large span construction [2]. Since Wangaard [3] and Biblis [4] first researched on reinforce wood with a synthetic fiber at the 1960s, there are extensive researches have been conducted to improve the mechanical properties of LVL reinforced with the synthetic fiber. In recent years, synthetic fiber reinforced LVL, as an important wood-based composite, has been extensively used in construction of residential, commercial buildings, container transportation and furniture et al. [5]. Considering its wide and attractive applications, how to obtain a fiber reinforced LVL composites with excellent mechanical performance, high load-bearing capacity and good durability have been widely researched [6]. However,

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due to the chemically inert surface and low surface energy of the synthetic fiber, the interfacial adhesion between fiber and adhesive is commonly unsatisfactory. To date, numerous surface modification methods have been employed to improve interfacial adhesion between fiber and matrix, including surface heat treatment [7], microwave [8], acid-based etching treatment [9] and coupling agent treatment [10]. Among them, the most common methods are the acid treatment and coupling agent treatment, for instance, Tran et al. [11] modified carbon fiber (CF) by boiling for 5 h in HNO3 solution, and He et al. [12] modified CF by immersing in the 1 mM silanes solution for 8 h at room temperature followed by washing with deionized H2O to clean the surface residues, drying at 110 °C for 10 min, ultimately cooling in a vacuum desiccator. There can be little doubt that the above-mentioned surface modification methods have positive effect on the performance of interfacial adhesion. Nevertheless, the physical or chemical processes involved in these methods usually are complex, time- and energy-consuming. Furthermore, these processes easily cause environmental pollution, and the base fiber treated by these developed methods are unsatisfactory in tensile strength. Therefore, a facile, cost-efficient and environmentally friendly surface modified technology for the improvement of interfacial adhesion is highly desired. For the past few years, rapid modification of material surface by plasma technology has been emphasized. Plasma technology has been accepted as an efficient, fast and pollution-free surface modification method, which can fast change the chemical and physical structure of surface layer, but without influencing on the properties of inner layers of materials [13]. Up to now, plasma modification has been successfully applied to modify various materials, including wood [14], crops [13] and polyethylene (PE) film [15]. Plasma is defined as an electrically conducting medium, generally consisting of radicals, free electrons, positively charged ions and neutral atoms or molecules or both, which means there is an ion bombardment and strong thermal effect during plasma modification [16], and that can be one possible reason for efficient modification of materials. Another reason is that plasma can efficiently modify material surface by improving the interaction of plasma components and materials [17]. Furthermore, different species of high-energetic components can be generated by changing plasma gases [18], these resultant components can strongly interact with material surface, thereby grafting plentiful functional groups on the material surface. Compared to other plasma technologies, airpressure-dielectric-barrier-discharge (DBD) plasma not only can avoid the utilization of vacuum equipment, but also enables continuous treatment for larger objects and products [19]. Recent literatures [20] showed that DBD air plasma modification can significantly improve the surface wettability in a short time, thus improving the interfacial adhesion performance effectively. Various studies have been reported in the literature by researcher who have made great progress in the synthetic fiber reinforced wood-based engineered products, but there is no study focus on the reinforcement of LVL with glass fiber fabric using the DBD air plasma surface modification technology. In this work, we applied DBD air plasma to modify GF surface to increase interfacial strength of GF reinforced LVL composites. The effect of plasma processing power on the surface morphology, chemical composition and the surface wettability of the GF were studied. The plasma enhancement effect was examined by performing the shear strength test of GF reinforced plywood, and the bending properties of the GF reinforced LVL composites under the optimal plasma processing power were further tested. In addition, massive active species generated from DBD air plasma were monitored and distinguished by the optical emission spectroscopy (OES), and the mechanism of interfacial adhesion between plasmamodified GF and adhesion were also systematically investigated.

This research aims to use DBD air plasma modification technology to increase interfacial adhesion between GF and matrix for the purpose of fabricating GF reinforced LVL, the prepared plasmamodified GF reinforced LVL composites by their virtues of outstanding mechanical properties is a promising structural material in wooden architectural application. 2. Materials and methods 2.1. Materials Commercial poplar veneers with dimension of 300  300  15 mm3 and phenol formaldehyde (PF) adhesive were purchased from Jinhu Hongda Wood Industry Co., Ltd., China, and Dare Wood-Based Panel Group, China, respectively. The moisture content of the veneers was adjusted to approximately 6% by keeping them at constant temperature and humidity for at least 7 d. The solid content and viscosity of PF adhesive was 47 ± 1% and 365 mPa.s, respectively. Commercial GF fabric (Eglass) (1500  800 mm2) was obtained from Toray, Japan. The consumption of woven GF was 500 g/m2, and it was woven using the ‘‘plain-weave” type of knitting. Basic properties are: filament diameter, 14 mm; number of standards per 1 kg, 3 million pcs; tensile strength of standards, 1700 MPa; Young’ s modulus of elasticity, 71 GPa; density, 2.5 g/cm3; and breaking elongation, 3%. 2.2. Plasma treatment The GF fabric was modified by means of the self-designed DBD air plasma experimental setup, as described by Zhou et al. [21]. The GF fabric was modified uniformly by passing a gap between two electrodes on a roller conveyor. The plasma processing rate was 8 m/min, and the plasma processing power was set to 1.5, 3, 4.5 or 6 kW. 2.3. Characterizations Field emission scanning electron microscopy (FE-SEM, JSM-7600) was used to observe the surface morphology of GF before and after plasma modification. For the FE-SEM images, SE detector was used and operating at 5 kV accelerating voltage and 25 pA current. Samples were subjected to a gold-coating process to reduce the charging effects before imaging. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Bruker Optics Inc) was performed to reveal the chemical structure of GF before and after plasma modification. All the FTIR data was collected from the wavenumber of 400 cm 1 to 4000 cm 1 with a spectral resolution of 4 cm 1 and averaged over 128 scans. X-ray photoelectron spectroscopy (XPS) was applied in GF before and after plasma modification to investigate the surface chemical compositions, performed on an ESCALAB 250 system (Thermo, USA) using an Al Ka X-ray source (1486.6 eV) operated at 50 eV. XPSPEAK Software (version 4.0) was applied to the spectrum of C1s to obtain the 4 Gaussian peaks which correspond to 4 carbon-related components. An Ocean Optic S2000 spectrometer in the range of 200–900 nm with a low resolution of 0.7 nm was applied to identify the excited active species generated from DBD air plasma in the discharge. The OES spectra of DBD air plasma was obtained at 10 mm from the edge of the capillary. Surface wettability of the GF was evaluated by analyses of contact angle and surface free energy via a contact analyzer (Dataphysics DCAT 21). Wettability of the GF was assessed by a method known as a sessile drop. Droplets of liquid with volume size of 5 mL were deposited on the surface of GF. The process that liquid droplet spreads on the GF surface was captured by a digital camera for every 50 ms. All measurements were repeated ten times on each sample to obtain statically relevant data; results were averaged. Table S1 lists the relevant surface energy parameters of the two testing liquids for analysis of surface free energy. 2.4. Fabrication of GF reinforced LVL composites The GF reinforced plywood was prepared under different DBD air plasma processing powers to determine the effect of processing power on the interfacial behaviors (as shown in Fig. S1a). The three-layered GF reinforced plywood samples were prepared as follows: 120 g/m2 PF adhesive were coated on each single layer of poplar veneer. The hot-pressing parameters for plywood, including the temperature of 130 °C, the pressure of 1.2 MPa, and the time of 1.5 min/mm. After the hot pressing, the plywood specimens were stored at the ambient temperature for 12 h prior to the shear tests. Shear strength tests were performed on a mechanical testing machine (HD-500, Shenzhen Sana Material Test Instrument Co., Ltd., China) at a cross-head speed of 10 mm/min in accordance with the China National Standard (GB/T 9846-2015) [22] for class I plywood (as shown in Fig. 6a). Twelve plywood samples (25  25 mm2) were cut from each plywood specimen and submerged in a preheated water bath at 100 °C for 3 h. The samples were cooled at room temperature for 10 min prior to the test. Based on the results of shear test, we adopt the approach of single layer GF cloth on the subsurface to fabricate the GF reinforced LVL composite at the optimum plasma processing power. Poplar veneer singlesided sizing amount of PF adhesive was 120 g/m2, and the hot press time, pressure,

W. Zhang et al. / Construction and Building Materials 233 (2020) 117315 and temperature were 1.5 min/mm, 1.2 MPa, and 130 °C, respectively. The plasmamodified GF reinforced LVL composite obtained at the optimum plasma processing power was denoted as strengthened LVL, and its counterpart without plasma modification was marked as control LVL (as shown in Fig. S1b). Three replicates were prepared for each type of the GF reinforced LVL composite, and the prepared specimens were maintained at room temperature for 48 h to relieve stress. According to the Chinese National Standard (GB/T 20241-2006) [23], the bending test (four-point and three-point bending test) was performed on a mechanical testing machine (HD500, Shenzhen Sans Material Test Instrument Co., Ltd., China), with a cross-head speed of 2.5 mm/min, a pre-load of 2 N and a span-to-depth of 12:1 (as shown in Fig. S2a and b).

3. Results and discussion 3.1. SEM analysis The surface morphologies of unmodified and plasma-modified GF were analyzed by SEM, and the results were shown in Fig. 1. From the high-resolution SEM images, the whole, macroscopically topographies of GF surface, and the etching induced by DBD plasma can be well revealed. Obviously, the pristine GF shows smooth and clear surfaces, while the well-resolved surface grooves of GF modified by 1.5 kW plasma can be observed in corresponding SEM image (Fig. 1a and b). The emergence of grooves can be attributed to the strong ion bombardment and thermal effect of DBD air plasma. As plasma processing power increased from 3 kW to 4.5 kW, the superficial grooves on GF surface became more obvious (as shown in Fig. 1c and d). The increased plasma processing power lead to much stronger ion bombardment and thermal effect on the GF surface, and thus generating larger areas of etching. Previous reports have confirmed that the surface rough to some extent help increase the surface liquid contact area and further contribute to the wettability of liquid on surface [20]. However, the further increasing plasma processing power cannot deepen or enlarge these grooves, which could be related to the exposure of new inert surface that induce by the excessive etching under high power plasma modification. Thus, the excessive etching of plasma will

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also reduce the surface roughness, which will affect the wettability of the liquid on the GF surface. 3.2. FTIR-ATR analysis The changes of surface functional groups induced by DBD air plasma modification were analyzed via FTIR-ATR spectroscopy and the results were displayed in Fig. 2. The broad peak at wavenumber of 3455 cm 1 corresponds to the OAH stretching band. Furthermore, the peaks at 1732, 1628 and 1398 cm 1 can be assigned to the C@O, OAC@O and C@C stretching bands, respectively [24]. Infrared spectrum of GF surface has obvious absorption peaks at 933 cm 1 and 706 cm 1, which can be assigned to antisymmetric stretching vibration and symmetric stretching of the characteristic peak Si-O-Si [25]. As shown in Fig. 2, no significant changes were found in the infrared spectrum of GF before and after plasma modification. Since the ATR technique suffer from limitation in effective depth penetration, particular in shortwavenumber region, resulting in that the variation of functional groups induced by the plasma modification cannot be evidenced by using infrared spectra. Thus, the XPS analysis was carried out for better understanding of chemical profile of GF before and after plasma modification. 3.3. XPS analysis XPS analysis was carried out to further analyze the elemental composition of the GF surface before and after DBD air plasma modification. Elemental composition and O/C ratio of the GF surface were summarized in Table S2. The unmodified GF surface has 74.38% of carbon and 19.56% of oxygen, which can be attributed to absorbed oxygen or surface oxidation during the manufacture of GF. Traces of other elements, namely nitrogen, silicon, were also determined on GF surface. After 1.5 kW plasma modification, the O/C ratio increased from 0.26 to 0.38, which can be

Fig. 1. SEM topographies of GF: (a) unmodified, (b) 1.5 kW, (c) 3 kW, (d) 4.5 kW, and (e) 6 kW.

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Fig. 2. FTIR-ATR spectra of GF: (a) unmodified, (b) 1.5 kW, (c) 3 kW, (d) 4.5 kW, and (e) 6 kW.

attributed to the grafting of oxygen-containing groups on GF surface. With the plasma processing power increased from 1.5 kW to 4.5 kW, the relative content of carbon decreased from 66.99% to 63.83%, and the relative content of oxygen increased from 25.75% to 29.75%, respectively. The composition changes of carbon and oxygen indicate the accumulation of oxygen during DBD air plasma modification. Also, a slight change of the silicon content can be observed after DBD plasma modification, which can be attributed to the Si-O-Si functional groups on GF surface induced by DBD air plasma [26]. However, the further increasing plasma processing power did not lead to the further enrichment of oxygen, which suggests that the proper plasma processing power is 4.5 kW. High-resolution XPS C1s spectra can reveal further information about carbon-related chemical species on GF surface and the results are shown in Fig. 3 and Table S3. Carbon-related bonds on GF surface can be divided into four types: CAC (284.8 ± 0.1 eV),

CAO/CAN (286.3 ± 0.1 eV), C@O (287.7 ± 0.1 eV) and OAC@O (289.0 ± 0.1 eV) [27]. As shown in Table S3, the CAC component (sp2 allotropic form) is the most abundant composition (54.30%) on the pristine GF surface. After 1.5 kW plasma modification, the relative percentage of CAC component decreased to 52.10%. Furthermore, it can be noted that the content of oxygen-containing groups (CAO, C@O and OAC@O) increased as plasma processing power increased from 1.5 kW to 4.5 kW. The peaks of C@O/OAC@O groups can be deconvoluted from the high-resolution C1s spectra of plasma-modified GF surface, as shown in Fig. 3. After 1.5 kW plasma modification, the C@O content and the OAC@O content showed a trend of increasing, while the further enhancing plasma processing power to 4.5 kW, the C@O content increased from 4.80% to 6.50%, and the OAC@O content increased from 1.50% to 4.30%, respectively. Such variation is more likely due to the strong oxidative free radicals along with neutral species and UV radiation in DBD air plasma. The concentration of the newly formed oxygencontaining functional groups, such as CAO, C@O, and OAC@O, increased after DBD air plasma modification, whereas the content of CAC with a tendency of decreasing. The changes in C1s composition indicates that GF surface was oxidized to oxygen-containing functional groups, and the surface wettability enhanced accordingly. However, further increasing power did not bring the improvement of the content of OAC@O groups. Conversely, the relative percentage of OAC@O component decreased from 4.30% to 3.20% with the power increased from 4.5 kW to 6 kW. This variation can be attributed to excessive etching of DBD air plasma at high plasma power. As mentioned above, high plasma processing power has high temperature and energy. Hence, the effect of excessive etching is much greater than the introduction of oxygencontaining functional groups on the GF surface under the high plasma processing power. Therefore, the proper DBD air plasma modification power should be carried out at 4.5 kW. 3.4. OES analysis In order to qualitatively analyze plasma composition in the experiment, we employed optical emission spectroscopy (OES) to verify the presence of active species in the DBD air plasma and

Fig. 3. Carbon peak fitting of the XPS narrow scan spectra of the GF: (a) unmodified, (b) 1.5 kW, (c) 3 kW, (d) 4.5 kW, and (e) 6 kW.

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Fig. 4. OES spectrum of the DBD air plasma afterglow (a) and schematics of the mechanism of DBD air plasma modification on the GF surface (b).

the results were displayed in Fig. 4a. It is clear that a lot of characteristic emission lines for different particle can be found, which belong to the four bands, namely N2 positive emission band (310–430 nm), O emission band (297 nm), OH radical (244 nm) and NO violet system (282 nm) [28]. The majority of emission lines can be observed from the N2 positive emission band, suggesting that majority of particles in DBD air plasma are nitrogen-based. One reason for this result is more likely to associate with the around 78% content of nitrogen in air. Additionally, the OH radical emission band can be attributed to the ionization of water vapor in air. Meanwhile, the presence of NO violet system implies the occurrence of reaction between different reactive plasma species. Possible reaction that generated OH radicals and NO violet system were shown in reaction 1–2 and 3–4 in Table S4, respectively [29]. In addition, the distinct emission lines observed at 778.7 nm and 844.6 nm can be assigned to atomic oxygen, which is the main category of oxygen-based particles in DBD air plasma. Due to the electronegativity of oxygen, it is unlikely to generate atomic oxygen through direct ionization of oxygen and electron. Owing to the presence of inert gases (argon and helium) in air, the reaction of collision ionization of oxygen molecular ion seems to be the main reaction to generate atomic oxygen, namely penning ionization [30] (as shown in reaction 5–6 in Table S4). Because the OH radicals and atomic oxygen have stronger oxidability and are easy to react with other active species, the direct oxidation could be occurred easily in the DBD air plasma. Even though the N-based particles is hard to oxidize GF surface directly, the kinetic energy is enough to lead to the cleavage of chemical

bond and generate abundant reactive sites. Therefore, postoxidation that was caused by the N-based particles occurred simultaneously with the direct oxidation of DBD air plasma. According to the above-mentioned analysis, the DBD air plasma surface modification mechanism of GF can be illustrated in Fig. 4b. 3.5. Contact angle and surface free energy Table S5 presents the results of water contact angle (WCA) of the GF surface versus DBD air plasma processing power. Furthermore, the PF adhesive contact angle and surface free energy of GF surface were also investigated and the results are shown in Fig. 5. Compared to the unmodified GF, the WCA decreased from 54.03° to 34.58° after 4.5 kW plasma modification. Also, the decrease in PF adhesive contact angle with the increasing of plasma processing power can be observed in Fig. 5a. After the 4.5 kW plasma modification, the equilibrium contact angle of PF adhesive decreased from 50.01° to 33.75° in comparison to that of unmodified GF, indicating that the plasma modification can significantly improve the wettability of adhesive on the GF surface. Such variation of WCA and PF contact angle on the plasmamodified GF suggest that DBD air plasma have a positive influence on the wettability of GF surface. The effect of different plasma processing power on the GF surface free energy were shown in Fig. 5b. Compared to unmodified GF, the polar component (cP S) of plasma-modified CF increased dramatically as well as the dispersion component (cD S) and total surface free energy increased, indicating that the activation of GF

Fig. 5. PF adhesive contact angle (a) and surface energy (b) of GF at different plasma processing power.

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surface after DBD air plasma modification. Compared to the pristine GF, the surface free energy increased from 51.73 mJ/m2 to 56.32 mJ/m2 after 1.5 kW plasma modification. The further improved surface free energy can also be obtained as plasma processing power increased to 4.5 kW. Meanwhile, the cP S dramatically increased from 30.9 mJ/m2 to 38.4 mJ/m2 with plasma processing power improved from 1.5 kW to 4.5 kW. Due to the high electronegativity of oxygen, these introduced oxygencontaining functional groups are highly polar, and thus resulting in the improvement of surface polarity of GF surface. In contrast, the cP S almost keep constant when the plasma processing power increased from 4.5 kW to 6 kW. It can be attributed to that the high plasma processing power weakened the effect of GF surface oxidation, which can also be examined by the analysis of WCA and PF adhesive contact angle tests. Based on the above analyses in our case have demonstrated that DBD air plasma modification is beneficial to graft oxygen-containing functional groups on the GF surface in a short time. It can be concluded that the surface hydrophilic of GF was enhanced by the plasma modification, which can be attributed to the rough and oxygen-doped fiber surface induced by DBD air plasma. 3.6. Shear strength test Evaluation of mechanical performance of GF reinforced plywood was carried out with aspect to shear strength. The influence of different plasma processing power on the shear strength of GF reinforced plywood were determined, as shown in Fig. 6b. Due to the inherent chemical inertness of GF surface, the unmodified GF reinforced plywood showed a poor shear strength (0.24 MPa), which is significantly below the requirement of the Chinese National Standard for plywood (GB/T 9846-2015: 0.7 MPa) [22]. In sharp contrast, a significant enhancement of shear strength can be obtained after DBD air plasma modification. The shear strength increased from 0.68 MPa to 1.13 MPa as the plasma processing power improved from 1.5 kW to 4.5 kW. However, the further increased plasma processing power did not lead to a satisfactory shear strength, it could be due to the excessive etching and the decline of oxygen-containing groups as proved by previous analysis. These results suggested that the proper plasma processing power is 4.5 kW. Digital image correlation (DIC) technology, an advanced noncontact, full-field optical metrology method, have been widely applied to analyze two-dimensional shear strain distribution [31]. In this experiment, DIC technique was employed to determine the shear strain distribution in shearing section of prepared GF reinforced plywood (as shown in Fig. 6c). It can be found that the shear strains in shearing section of prepared GF-reinforced plywood were comparably low, and there is no obvious strain concentration under the load of 100 N. Further increasing load from 100 N

to 300 N, a significant difference appeared on the strain distribution in shearing section of GF-reinforced plywood. The strain was obviously concentrated at both ends of shearing section and became gradually smaller towards to the middle area in shearing section. The strain concentration was more pronounced for the unmodified GF-reinforced plywood specimens. In contrast, the strains throughout the shearing section of the plasma modified specimens are much more uniform than the unmodified one, indicating that the plasma modification is beneficial to reduce stress concentration. These results suggest that DBD air plasma modification had an appreciable influence on the strain distribution in shearing section. The phenomenon of strain concentration gradually decreased under the same load with the plasma processing power increased from 3 kW to 4.5 kW, which demonstrated that the DBD air plasma modification can effectively reduce stress concentration. However, the further increased power (6 kW) did not lead the further reduction, the strains in both ends of the specimen were significantly larger than those in other places, demonstrating the presence of strain concentration and fine cracks have gradually occurred. This behavior can be explained by the influence of plasma processing power on the performance of shear strength. The obtained results suggested that DBD air plasma modification was beneficial to improve strain transfer across the shearing section, which may contribute to an increase in the shear strength of GF reinforced plywood. 3.7. Fractographic analysis Fractographic analysis of the GF-reinforced plywood at different plasma processing power was evaluated by the SEM micrographs, and the results were shown in Fig. 7. As shown in Fig. 7a, it can be seen that most of GFs showed bare fracture surface without the residues of the adhered resin, and the delaminated resin residues are completely separated from the fibers. Interfacial debonding is obviously observed and there is no resin adhered on the fiber surface, which shows that the interfacial adhesion between the untreated fiber and resin is weak [32]. In contrast, for the fibers covered with resin after plasma modification, the interfacial adhesion has been obviously improved and strong fiber-matrix bonding could be observed. The extent of resin cover of plasma modified GFs was increased after 1.5 kW plasma modification (as shown in Fig. 7b), and the further increase in the extent of covered resin on GF surface can also be found when the plasma processing power increased from 1.5 kW to 4.5 kW. Particularly, some broken GFs were obviously observed from the samples after 4.5 kW plasma modification (as shown in Fig. 7d), which can be attributed to the high shear stress transferred from the matrix to fiber, as a result of the strong interfacial adhesion [33,34]. However, the further increased plasma processing power bring about the

Fig. 6. Tensile shear strain measurement (a), analysis of the shear strength (b) and the tensile shear strain distribution (c) of GF-reinforced plywood at different plasma processing power.

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Fig. 7. SEM micrographs of GF-reinforced plywood at different plasma processing power: (a) unmodified, (b) 1.5 kW, (c) 3 kW, (d) 4.5 kW, (e) 6 kW.

unsatisfactory changes that the bare fibers surface is exposed, and the residual resin exists independently of the fiber (Fig. 7e). This may be due to the excessive etching of strong ion bombardment under the high plasma processing power. This result indicates that the optimal plasma processing power is 4.5 kW. Comparing to the fractographic morphologies of unmodified and plasma modified samples, we could find that the fracture model of the composites changes from pure interfacial failure to combination failures of interface and resin interlayer.

uniformity of sizing, difference in poplar veneer and accuracy of test set-up. Nonetheless, there are litter doubt that DBD air plasma technology, as an efficient, low-cost and environmentally friendly surface modification method, can be applied to enhance the interfacial adhesion between GF and matrix of GF reinforced LVL composites and obtained excellent mechanical performance to broaden the application of wooden structural building material.

4. Conclusion 3.8. Bending test of GF reinforced LVL composites. Bending test was carried out to analysis the flexural behavior of GF reinforced LVL composites prepared from plasma-modified GF at optimum plasma processing power (4.5 kW). According to Chinese National Standard (GB/T 2024-2006) [23], we adopted fourpoint and three-point bending test to determine flexural strength of GF reinforced LVL composites (as shown in Fig. S2a and b), and the results of modulus of rupture (MOR), modulus of elastic (MOE) and horizontal shear strength were listed in Fig. S2c–e, respectively. The strengthened LVL exhibited better flexural performance than the control LVL under the two types of load, vertical (\) and horizontal (//) load, which indicated that GF surface plasma modification at 4.5 kW have a positive influence on the bending performance of GF reinforced LVL composite. Specific to see, the MOR\ and MOR// value of the strengthened LVL, the percentage increase to that of control LVL was 15% and 36%, respectively. Meanwhile, compared to the control LVL, the MOE\ value and the MOE// value of the strengthened LVL increased by 3.6%, 16.4%, respectively. Furthermore, the horizontal shear strength correspond to the two types of load were also improved. The obtained results suggest that a satisfactory flexural performance of GF reinforced LVL composites can be acquired by the DBD air plasma modification on the GF surface. Even though there are existed other factors that influenced the test results, such as: the

In this study, DBD air plasma was successfully applied to improve the interfacial strength between GF and PF adhesive of GF reinforced LVL composites. Based on the analysis of the data, the obtained conclusions and observations are as follows: (1) The massive active species (OH, NO and O) generated from the DBD air plasma not only have significant surface etching effect, but also induced multiple oxygen-containing polar functional groups (i.e., carbonyl, and carboxyl) grafting on the plasma-modified GF surface; (2) Compared to the unmodified GF, the surface free energy increased by 17.68% and PF contact angle decreased by 32.5% for the 4.5 kW plasma-modified GF, and the further increased plasma processing power cannot exhibit a positive effect; (3) The shear strength reached maximum for GF reinforced plywood treated by 4.5 kW plasma, and the resulting composite exhibited relatively uniform strain distribution; (4) Since the synthetic effect of plasma etching and polar groups grafted on the GF surface, the interface adhesion between the GF and PF adhesion was improved for the 4.5 kW plasma modified GF reinforced LVL composites, thus resulting in the MOR and MOE values increased by 36% and 16% in comparison to the unmodified specimens.

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