Improvement of the Performance of Plantation Wood by Grafting Water

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Improvement of the Performance of Plantation Wood by Grafting Water-Soluble Vinyl Monomers onto Cell Walls Hong-Bo Qiu, Sheng Yang, Yanming Han, Xiao-Shuang Shen, Dongbin Fan, Gaiyun Li, and Fuxiang Chu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03112 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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ACS Sustainable Chemistry & Engineering

Improvement of the Performance of Plantation Wood by Grafting Water-Soluble Vinyl Monomers onto Cell Walls Hongbo Qiu,†, ‡ Sheng Yang,§ Yanming Han,‡ Xiaoshuang Shen,‡ Dongbin Fan,‡ Gaiyun Li,*, †, § Fuxiang Chu*, §

†

Research Institute of Forestry New Technology, Chinese Academy of Forestry, NO.1

Dongxiaofu Xiangshan Road, Haidian District, 100091, Beijing, China.

‡

Key Laboratory of Wood Science and Technology of National Forestry and Grassland

Administration, Research Institute of Wood Industry, Chinese Academy of Forestry, NO.1

Dongxiaofu Xiangshan Road, Haidian District, 100091, Beijing, China.

§

Hunan Collaborative Innovation Center for Effective Utilizing of Wood & Bamboo

Resources, Research Institute of Wood Industry, Chinese Academy of Forestry, NO.1 Dongxiaofu Xiangshan Road, Haidian District, 100091, Beijing, China.

*Corresponding author: Tel.: +86-10-62889433; Fax: +86-10-62889433;

Address: NO.1 Dongxiaofu Xiangshan Road, Haidian District, 100091, Beijing, China.

Email: [email protected] (G. Y. Li), [email protected] (F. X. Chu). 1

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ABSTRACT: The accomplishment of lightweight reinforcement of wood through an environmentally friendly approach has being seen as a hard nut to crack for a long time. In this study, a novel method which accomplished by inserting water soluble vinyl monomers into wood cell walls followed by in-situ polymerization was proposed. The distributions of the modifiers in the wood cell walls and the graft mechanism between water-soluble vinyl monomers and wood cell wall components were thoroughly investigated. Results indicated that the water-soluble vinyl monomers could effectively permeate into the wood cell walls and connected with the hydroxyl groups of the lignin and polysaccharides via covalent bonds. The lumen of the wood kept unfilled during the modification process, and just a 40% weight gain was achieved. However, a 68.37% anti-swelling efficiency (ASE) was achieved after a water-soluble vinyl monomer treatment under the optimized condition (when 20 wt% of NMA was co-added). The modulus of rupture (MOR) and modulus of elasticity (MOE) of the corresponding material increased 20.2% and 38.0%, respectively, compared with those of the untreated one.

KEYWORDS: Water-soluble vinyl monomers, Grafting, Cell walls, Plantation wood

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INTRODUCTION As one of the most abundant biomaterials, wood has attracted growing research interests in both academia and industries.1-2 High-quality solid timber has been widely used in construction and furniture for thousands of years due to its high strength-to-weight ratio, renewable and environmentally friendly nature.3 However, high-quality wood is now facing diminished supply due to low growth rate and stricter environmental regulations worldwide.4 Plantation forestry has been considered as an alternative source of wood products due to its rapid growth and low cost. However, some undesirable inherent shortcomings of the fast-growing plantation wood, including poor mechanical strength, unsatisfactory durability and poor dimensional stability, significantly limit its applications.5-6

In order to overcome the shortcomings of plantation wood mentioned above, various chemical modification methods have been proposed including esterifications,7 etherifications,8 silylations,9 in-situ polymerization of monomers,10 and impregnation with thermosetting resin.11 The improvement of the mechanical properties and dimensional stability of the wood was achieved by the reduction of the hydrophilic -OH groups of the cell wall components or the filling of the pore in the wood.12-13

The cell lumen filling accomplished by the free-radical polymerization of vinyl-type monomers has been confirmed as an excellent modification method of wood. The mechanical performances, dimensional stability, and biodeterioration resistance of the

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modified woods can be significantly improved compared with untreated ones.14-15 A product with trade name of ‘Neswood’ has been launched and used as parquet tiles or strips of parquetry.16 The employed monomers for wood treatment include methyl methacrylate (MMA),5 styrene (St),17 glycidyl methacrylate (GMA), 18 ethylene glycol dimethacrylate (EGDMA),19 and vinyl acetate (VAc).20 However, most traditional vinyl monomers are non-polar, making them difficult to penetrate into the wood cell walls and react with the hydroxyl groups of the wood matrix. Therefore, volatile and toxic organic solvents such as acetone, pyridine, and tetrahydrofuran have to be used to solubilize these non-polar monomers and guarantee their permeation in the wood cell walls. And the polymerization mainly occurs in the cell lumen,21 which can limit the effect on improving the dimensional stability, durability and mechanical properties of the modified wood. The interfacial compatibility between the wood phase and polymer phase is also a problem that needs to be solved. 22-23 In addition, the density and production cost of modified wood would be increased by filling of the cell lumen in the wood. It is therefore of pressing urgency to search for greener polymerization methods to not only improve the performance of wood, but also reduce negative environmental impact.

2-hydroxyethyl methacrylate (HEMA) is a water-soluble vinyl monomer. This vinyl monomer has adequate biocompatibility, hydrophilicity, swelling properties, and permeability.24-25 Up to now, poly-HEMA has been successfully used in various biomedical applications (i.e. as component of dental adhesives, in soft contact lenses, 5



vascular grafts, soft tissue substitutes, and drug delivery systems).26-28 N-methylol acrylamide (NMA) is also water-soluble vinyl monomer that is bi-functional possessing both vinyl and hydroxyl functionality and reactions may be carried out at either group independently. This allows the polymerization with other vinyl monomers and leaves the hydroxyl groups available as postcrosslinkers.29 For this reason, the modifiers prepared with HEMA and NMA was proposed to improve the mechanical properties and dimensional stability of plantation wood as the traditional vinyl monomers does. Upon the modifiers penetrating into the wood cell walls and the covalent bonds subsequently formed between the cell wall components and modification agent,30 the performances of the modified wood are expected to be significantly improved only by the reinforcement of the cell wall structure, thus both preventing the modified wood from overfilling of cell lumen and enhancing the interfacial compatibility between the wood phase and polymer phase. The permeation property and functionary mechanism of the modifiers in the wood cell walls were thoroughly analyzed. The distributions of the modifiers in the wood cell walls were monitored by scanning electron microscopy (SEM). The mechanisms of the graft reaction between water-soluble vinyl monomers and wood cell wall components were investigated using nuclear magnetic resonance (NMR) technique. The relationship between modifier formula and the performance of the modified wood were also studied.

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MATERIALS AND METHODS Materials. Poplar wood (Populus euramevicana cv.I-214) was collected from Sun jiazhuang forest farm, Yi County, Hebei province, China. Analytical grade D-cellobiose was purchased from Sinopharm Chemical Reagent co., Ltd. Enzymatic hydrolysis lignin (EHL) was received from Hong Kong Laihe Biotechnology Co., Ltd. Analytical grade HEMA was purchased from Beijing Jintongletai Chemical Products co., Ltd. Chemical grade NMA, analytical grade calcium chloride, and hydrogen peroxide (30% aqueous solution) were purchased from Sinopharm Chemical Reagent co., Ltd. All chemicals were used as obtained without further purification.

Modification of wood with water-soluble vinyl monomers. The modifiers were prepared with varied HEMA to NMA ratios. A 40 wt% aqueous solution of modifier was prepared for subsequent impregnation. CaCl2 and H2O2 were used as initiator for the grafting of vinyl monomers and wood cell wall components. The formulations of various impregnation solutions are shown in Table 1.

During the impregnation process, the wood samples were subjected to a -0.1 MPa vacuum for 30 minutes and then applied pressure at 0.9 MPa for 3 h. Subsequently, the pressure was released and the samples were air-dried at room temperature for 4~5 days.

After air-drying, the samples were dried in an oven. Firstly, the samples were heated at 40 °C for 24 h to make the water-soluble vinyl monomers to be grafted onto the cell 7



wall matrix. Secondly, the samples were heated at 80 °C for 12 h to complete the polymerization. Finally, all samples were dried at 103±2 °C until a constant weight was obtained.

Graft copolymerization of D-cellobiose and enzymatic hydrolysis lignin (EHL). The HEMA (24 g) and NMA (6 g) were dissolved in 45 g of deionized water in a 100 ml beaker. Then, 0.6 g of CaCl2 and 0.6 g of H2O2 were added into the beaker, and the solution was stirred for 3 min until a transparent solution were formed. After that, 1 g of D-cellobiose or EHL was added in the mixture, and the beaker was immersed in a water bath to initiate grafting reaction at 40 °C with magnetic stirring (500 rpm) for 24 h. Grafted D-cellobiose was freeze-dried, precipitated in 20-fold acetone, filtered, and then freeze-dried to collect the modified copolymer. Grafted EHL was precipitated in 20-fold diluted hydrochloric acid solution (pH=3.0∼3.5), filtered, repeatedly washed with deionized water, and then freeze-dried to obtain the modified copolymer.

Preparation of polymer films. The modifier solutions prepared according to the formulation listed in Table 1 were poured into the polytetrafluoroethylene (PTFE) mold. Then, the PTFE mold was placed in a 50 °C oven to prepare films. The obtained films were removed from the mold and then heated at 103 °C to a constant weight. Finally, the crosslinked polymer film with a thickness of 4 mm was obtained.

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CHARACTERIZATION Microstructural morphology analysis. The microstructural morphology of both the untreated and treated wood sample was characterized by scanning electron microscopy (SEM, Hitachi S-8010, Japan), observed at a voltage of 15 kV. In addition, energy dispersive X-ray spectroscopy (EDS, EX-350, Horiba Scientific, Japan) was employed to analyze the distribution of carbon, oxygen and nitrogen in samples. Nuclear magnetic resonance spectroscopy (NMR) analysis. Both 1H and 13C NMR spectra of the grafted D-cellobiose and EHL samples were acquired with a nuclear magnetic resonance spectroscopy (NMR, Bruker-AVIII-400 MHz, Germany). The 30 mg sample was added to 0.5 ml DMSO-d6 and the resulting solutions were agitated until they had become homogeneous. The samples were then transferred into 5 mm NMR tubes. The sample tubes were capped with plastic lids and the tops were wrapped in parafilm. The NMR analyses of the samples were conducted according to the method described by a previous study.31

Polymer film performance evaluation. The performance of HEMA-NMA polymer films was evaluated according to previous study.32 The dried polymer films were cut into 20 mm × 20 mm × 4 mm size. The differences of the polymer films in weight and volume before and after a 24 h soaking in deionized water at room temperature were measured through a drainage method. Each measurement was conducted 3 times and the average values were used in further calculation. The water absorption (A) and

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swelling ratio (R) were calculated according to the following formula:

A (%) = (M2-M1) /M1×100

(1)

R (%) = V2/V1×100

(2)

Where, M1 represents the weight before soaking, and M2 is the weight after soaking. V1 and V2 represent the volume of the before and after soaking, respectively. Wood performance evaluation. Weight percent gain (WPG) was calculated from absolute dry masses before and after treatment. Eight replicates were used for each parameter in the tests. The modulus of rupture (MOR) and modulus of elasticity (MOE) of wood samples were measured in light of the china national standard testing methods “Method of Testing in Bending Strength of Wood” (GB/T 1936.1-2009) and “Method for Determination of the Modulus of Elasticity in Static Bending of Wood” (GB/T 1936.2-2009).

Measurements of mass loss (ML), water uptake (WU), and anti-swelling efficiency (ASE) were carried out with 10 replicates of cubic samples with a size of 20 mm × 20 mm × 20 mm (longitudinal × radial × tangential). The initial masses and volumes (oven-dried state) of the modified and untreated samples were determined, and then the cubes were soaked in water until constant volume was obtained. Weights and dimensions of the wet cubes were measured and then the samples were dried at 65 °C until a constant mass was obtained. The dimension and weight values of the obtained dry samples were also collected. The above procedure was repeated for four cycles. 10


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The ASE values for multiple water soaking cycles were calculated according to the reported method.33 For the calculation of ASE, the volumetric swelling coefficient of the modified samples were compared to that of the untreated samples after the first soaking cycle, which gives an indication of the efficiency of the treatment on the dimensional stability. The equations for ML and WU calculation are given below:

WU (%) = (W2-W1)/W1×100

(3)

Where, W1 represents the weight of sample before each cycle of soaking, and W2 is the weight of sample after each cycle of soaking.

The mass loss was calculated by the following formula:

ML (%) = (Wa-Wb)/Wa×100

(4)

Where, Wa represents the oven-dry weight of samples before each cycle of soaking, and Wb is the oven-dry weight of samples after each cycle of soaking.

RESULTS AND DISCUSSION

Polymer distribution in wood. To determine the spatial distribution of the modifier at cell well the cross-sectional microphotographs of both treated and untreated wood were collected. SEM images showed that the cell wall lumens were empty for both untreated (Fig. 1a) and treated (Fig. 1b) wood. However, the thickness of the treated wood cell walls was obviously larger than that of the untreated one. According to the 11



Nano Measure software statistics, the average thickness of the untreated cell walls was 3.02±0.69 µm (Table. S1), and the average thickness of the treated wood cell walls was 4.25±0.92 µm (Table. S1). At the same time, the microcracks between the treated wood cell walls at the middle lamella were filled with the modifier (marked by white arrows in Figure 1a and 1b).

The SEM-EDS images was further collected to show the distributions of C, O, and N (originate from HEMA and NMA) in the cross section of the wood samples. N element was detected in the treated wood cell walls with a content of 13.76% (Fig. 1d). However, almost no N element was detected (Fig. 1c) in the untreated one. This result indicated that the water-soluble vinyl monomers have been successfully inserted into wood cell walls during the modification process. 1

H NMR and 13C NMR analyses. To explain the mechanism of the grafting reaction

between water-soluble vinyl monomers and wood cell wall components, cellulose and EHL were treated with the modifier. The modified products were analyzed through NMR technique. Cellulose is a macromolecular substance with complicated structure, which is difficult to dissolve in deuterated reagents and cannot be analyzed by liquid NMR. The solid-state 13C NMR spectra of untreated and treated cellulose have no obvious difference (Fig. S1). This may be attributed to the low resolution of solid-state NMR. Therefore, D-cellobiose was selected as model compound of cellulose. This model compound was treated with the modifiers, and the modified products were analyzed through liquid NMR technique. The remained water-soluble 12


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vinyl monomers in the treated D-cellobiose and EHL have been removed before analysis. The signals in the NMR spectra of the samples were assigned according to previous articles.34-39 The 1H NMR spectra of untreated and treated D-cellobiose are shown in Fig. 2a and 2d, respectively. The peak at 4.97 ppm was corresponded to -OH (C2’) in D-cellobiose. It could be found that the intensity of the peak of -OH protons at C2’ position in the spectrum of the treated D-cellobiose was lower than that in the spectrum of the untreated one. This may be due to that the hydrogen of -OH (C2’) in D-cellobiose could be captured by the chlorine atom provided by initiator system and a D-cellobiose free radical will be generated. The water-soluble vinyl monomers were assaulted by the D-cellobiose free radical to initiate a graft copolymerization between them. The peaks at 1.28 and 2.07 ppm of the Fig. 2b were assigned to -CH2-CH- and -CH2-CH- of poly-NMA, respectively. The presence of these two signals indicated that the NMA has been grafted to D-cellobiose through the dehiscence of double bonds. The 13C NMR spectra of untreated and treated D-cellobiose are shown in Fig. 2c and 2d, respectively. The signals of typical D-cellobiose (C1 at 103.58 ppm, C4 at 70.49 ppm, and C6 at 61.48 ppm) could be observed in the Fig. 2c. The signals between 77.20 and 73.76 ppm were assigned to C2, 3, 5 of D-cellobiose. As can be seen in Fig. 2d, some new signals were observed in the 13C NMR spectra of treated D-cellobiose. It was clear that the -CH2-CH- (at 31.18 ppm) signal of poly-NMA could be observed in the spectra of the treated D-cellobiose. The graft copolymerization of NMA with D-cellobiose was confirmed by this phenomenon.

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The untreated and treated EHL were analyzed through NMR technique. The signals in the NMR spectra of the samples were assigned according to previous articles.40-44 The 1

H NMR spectrum of untreated EHL is shown in Fig. 3a. The chemical shifts at

7.6-5.8, 4.8-3.0, and 2.6-0.7 ppm were belonged to the protons in aromatic ring, methoxy groups, and aliphatic side chain groups in EHL, respectively. For the treated EHL (Fig. 3b), the peaks at 0.82 and 1.29 ppm were corresponded to -CH3 and -CH2-C- structures of poly-HEMA, respectively. This result indicated that HEMA has been grafted to EHL through the dehiscence of double bonds. The mechanism of the graft reaction between HEMA and EHL was consistent with the graft copolymerization of NMA onto D-cellobiose. The 13C NMR spectrum of untreated EHL is shown in Fig. 3c. The signals around 110-160 ppm belonged to the carbon atoms on the benzene ring. The peak at 56.36 ppm was attributable to the methoxy proton of EHL. As shown in Fig. 3d, the signals of typical HEMA were observed in the spectrum of treated EHL. The peaks at 44.61 and 56.50 ppm corresponding to quaternary carbons and CH2-C- structure of poly-HEMA could be observed. This result was consistent with that obtained through 1H NMR analysis.

Lignin is a macromolecular substance with complicated structure; and its grafting reaction site is difficult to be determined. Therefore, 2-phenylethyl alcohol, 2, 6-dimethoxyphenol, and 4-n-propylphenol were selected as model compounds of EHL. These model compounds of lignin were treated with the modifiers, and the modified products were analyzed through NMR technique. As shown in Fig. S4a, the 14


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chemical shift of signal at 5.34 ppm belonged to the phenolic hydroxyl proton in 2, 6-dimethoxyphenol. However, this peak was not observed in the spectrum of the treated 2, 6-dimethoxyphenol (Fig. S4b), implying that the phenolic hydroxyl proton was consumed during the modification process. The signals at 0.78 and 1.24 ppm belonging to -CH3 and -CH2-C- structure of poly-HEMA, respectively, were found in the 1H NMR spectrum of treated 2, 6-dimethoxyphenol. Some new signals (-CH3 at 17.37 ppm, quaternary carbons at 45.50 ppm, and -CH2-C- at 57.26 ppm) of poly-HEMA were also observed in the 13C NMR spectrum of treated 2, 6-dimethoxyphenol. This result indicated that HEMA has reacted with 2, 6-dimethoxyphenol by graft copolymerization (dehiscence of double bonds). The NMR analyses of model compounds (Fig. S2-S4) showed that the reaction sites were mainly at the position of phenolic hydroxyl groups in EHL.

Properties of the polymer films. Fig. 4 illustrates the surface morphology of polymer films with different NMA dosages before and after soaking in water. The volume data of the treated and untreated polymer films have also been provided in Fig. 4. The polymer films without the addition of NMA were obviously swollen after soaking. When 5 wt% of NMA was co-added, the surface of the polymer film peeled off after soaking in water for 24 h. The dimension differences between the polymer films before and after soaking was obviously decreased by increasing the addition amount of NMA in the modification system. This result indicated that the NMA could improve the water resistance of the polymer film. 15



Table 2 also provides information about the water absorption (A) and swelling ratio (R) changes of the polymer films after soaking treatment. The A values decreased from 41.68 to 4.25% and the R values decreased from 182.25 to 102.25%. As shown in Table 2, when 20 wt% of NMA was co-added, the water absorption of the polymer film decreased to 8.22%. With the increase of NMA content, the water absorption of the polymer film decreased slowly. When 50 wt% of NMA was co-added, the water absorption of the polymer film increased to 5.28%. Actually, in the preparation of polymer films, the presence of double bond and hydroxyl group in both NMA and HEMA could undergo copolymerization and crosslinking reactions.45 The formed three-dimensional network structure could improve the cohesive strength of the polymer film. Therefore, NMA could provide water resistance and enhance the strength of the prepared composites.

Improved wood properties. Fig. 5 shows the physical properties of the treated wood samples. When the water-soluble vinyl monomers (HEMA and NMA) concentration was 40 wt%, the WPG (about 40%) of the treated wood kept almost constant with the increase of NMA content (Fig. S5). This indicated that the proportion of NMA had little effect on the WPG of the treated sample. The weight loss of the samples during repeated soaking cycles was caused by the removal of extractives and non-reacted water-soluble vinyl monomers. The mass loss values of the different samples after the water soaking cycles can be seen in Fig. 5a. The extractives of untreated samples were removed after two water soaking cycles, but for the treated wood samples, those 16


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non-reacted water-soluble vinyl monomers were mostly removed after three water soaking cycles.

WU values of the samples were measured to evaluate the water repellency of the modified wood upon water soaking cycles (Fig. 5b). The results showed that the WU values of all treated woods were lower than the untreated one. With the increase of NMA content, the WU values of the treated woods decreased. When 50 wt% of NMA was co-added, the WU value of the treated wood was reduced by almost half of that of the untreated wood. It was attributed to the permeation of HEMA-NMA into wood cell wall and their polymerization with the cell wall components after curing, which occupation of flow path originally available for water and impeded water uptake. However, after water soaking cycles, the extractives and non-reacted water-soluble vinyl monomers in the modified wood samples were removed, which make the cell wall more accessible to water.

The ASE was also measured to evaluate the dimensional stability of the treated wood upon water soaking cycles (Fig. 5c). The sample with higher ASE value possessed better dimensional stability. When HEMA was used as modifier, the ASE of the treated wood was 47.88% after first water immersion cycle. When 20 wt% of NMA was co-added, the ASE of the treated wood could reach up to 68.37% after first water immersion cycle. It was also attributed to the permeation of HEMA-NMA into wood cell wall and their polymerization in the cell walls after curing. According to the previous analysis, the vinyl monomers could graft onto the surface hydroxyls of wood 17



cell walls to form polymers, which was expected to bulk the cell wall and occupation of flow path originally available for water. The ASE values of treated wood decreased with the increasing of soaking cycles. The decrease of ASE values may be explained by the removal of extractives and non-reacted water-soluble vinyl monomers, which make the cell wall more accessible to water. Even so, the treated wood (when 20 wt% of NMA was co-added) still showed an ASE value above 40% after four soaking cycles, which indicated that the modification method proposed in this study could effectively improve the anti-swelling ability of wood. After the water soaking cycles, the water-soluble vinyl monomers treated wood showed better anti-swelling efficiency than the traditional vinyl monomers treated one.22

The change of MOE and MOR of the treated wood along with the increasing of the NMA dosage are given in Fig. 6. The MOR of the treated wood increased from 62.27 MPa to 81.85 MPa as a function of NMA dosage. Compared with the untreated wood (66.02 MPa), the MOR of treated wood (62.27 MPa) only by HEMA was slightly decreased. When 20 wt% of NMA was co-added, the MOR of the treated wood increased by 20.19% compared with the untreated one. With the increase of NMA content, the mechanical properties of the treated wood increased slowly. On the other hand, when the dosage of NMA was 50 wt%, the maximum value (81.85 MPa) of MOR was achieved. With the increasing of dosage of NMA from 0% to 50 wt%, the MOE of treated wood increased from 11.91 GPa to 14.40 GPa. When 20 wt% of NMA was co-added, the MOE of the treated wood increased by 38.0% compared with 18


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the untreated one. This improved strength could be explained by the restricted unwinding of cell wall micro-fibrils by polymer in the wood cell walls. HEMA-NMA treatment reduced the freedom of movement among polysaccharides in cell wall matrix, which increased the stiffness of wood. This treatment had negative effect on impact strength, but positive effect on modulus of elasticity. With the increasing of the NMA dosage, the presence of double bond and hydroxyl group in both NMA and HEMA could undergo copolymerization and crosslinking reactions. HEMA and NMA formed of a three-dimensional network polymer in the cell walls and enhanced the strength of the prepared wood composites. When compared to 39.4% WPG of wood-methyl methacrylate composite, the water-soluble vinyl monomers treated wood presents almost similar MOR. However, the MOE of the modified wood in present work was significantly higher (17.08%) than those of the wood-methyl methacrylate composite.46 When the treated wood exhibits a WPG about 40%, 280 kg of water-soluble vinyl monomers was required for the treatment of 1 m3 wood. However, under the same WPG condition, 660 kg of methyl methacrylate were required for the treatment of 1 m3 wood.47,48 The modification method proposed in this study requires fewer monomers than the traditional vinyl monomer modification process does. For these reasons, the technique proposed in this study could be a new way for the green and cheap modification of wood.

The incorporation mechanism of water-soluble vinyl monomers with wood cell wall. The modification route (Fig. 7) was proposed according to the above analyses. 19



Schematic illustrating the grafting polymerization on wood was depicted in Fig. 7a and b, showing that the vinyl monomers can graft onto the surface components of wood (lignin and polysaccharides) to form polymers by HEMA and NMA. In unmodified wood, numerous hydroxyl groups are present on the surface of cellulose microfibrils, hemicellulose and lignin, which can serve as reacting sites for the free radical initiated polymerization (Figure 7c). The CaCl2-H2O2 was selected as polymerization initiator owing to its catalytic activity. The hydroperoxide and chloride ion reacted to form chlorine radical. The chlorine atom then abstracted hydrogen from hydroxyls of wood cell wall components to form the free-radical site and initiated polymerization. It should be mentioned that the reaction may be a concerted reaction between a hydroperoxide-chloride ion complex and cell wall components rather than the separate reaction steps as shown in Fig. 7d. On the one hand, the vinyl monomers could graft onto the phenolic hydroxyl groups in EHL and hydroxyl (C2’) groups in polysaccharides. On the other hand, NMA is a bi-functional molecule with an ethylene group and a hydroxymethyl group. The ethylene group could take part in the formation of polymer and hydroxymethyl group to reacted with the hydroxyl groups of the cell wall components. Therefore, NMA worked as a crosslinker between wood cell wall and polymer, and a three-dimensional interpenetrating network could be formed by the combination of the HEMA/NMA hybrid layer and NMA. The feasibility of a wood modification method was established through the reinforcement of the cell walls by the polymerization of HEMA and NMA and their reaction with

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cell wall components (Fig. 7b).

CONCLUSIONS In this work, an environmentally friendly wood modification method was established through the reinforcement of the cell walls by HEMA and NMA which are water-soluble vinyl monomers. The water-soluble vinyl monomers could effectively permeate into the wood cell walls and polymerized to form a rigid three-dimensional network with the initiating of CaCl2-H2O2. More importantly, the water-soluble vinyl monomers could be connected with the hydroxyl groups of the wood cell wall components via covalent bonds. A light weight gain of the plantation wood could be realized through the filling of cell walls and intercellular space after the treatment process proposed in this study. A 68.37% ASE was achieved under the optimized conditions. The MOR and MOE of the modified wood increased 20.2% and 38.0%, respectively, compared with those of the untreated one. The technique proposed in this study could be a new way for the green modification of wood.

AUTHER INFORMATION

Corresponding Author *G. Li. E-mail: [email protected] 21



*F. Chu. E-mail: [email protected]

Notes

The authors declare no competing financial interest

ACKNOWLEDGEMENTS The authors are very grateful for financial support from the National Key Research and Development Program of China (2017YFD0600203).

SUPPORTING INFORMATION

Supporting Information Available: Table S1 Statistical data of the untreated and treated wood cell wall thickness. Fig. S1 NMR spectra of cellulose, (a) 13C NMR spectrum of untreated cellulose; (b) 13C NMR spectrum of treated cellulose. Fig. S2 NMR spectra of β-phenylethyl, (a) 1H NMR spectrum of untreated β-phenylethyl; (b) 1

H NMR spectrum of treated β-phenylethyl; (c) 13C NMR spectrum of untreated

β-phenylethyl; (d) 13C NMR spectrum of treated β-phenylethyl. Fig. S3 NMR spectra of 4-n-propylphenol, (a) 1H NMR spectrum of untreated 4-n-propylphenol; (b) 1H NMR spectrum of treated 4-n-propylphenol; (c) 13C NMR spectrum of untreated 4-n-propylphenol; (d) 13C NMR spectrum of treated 4-n-propylphenol. Fig. S4 NMR spectra of 2, 6-dimethoxyphenol, (a) 1H NMR spectrum of untreated 2, 22


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6-dimethoxyphenol; (b) 1H NMR spectrum of treated 2, 6-dimethoxyphenol; (c) 13C NMR spectrum of untreated 2, 6-dimethoxyphenol; (d) 13C NMR spectrum of treated 2, 6-dimethoxyphenol. Fig. S5 WPG of wood treated with different addition of NMA.

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Figure captions Fig. 1 Morphologies of untreated and treated wood samples: cross-section of wood treated with 40 wt% (HEMA: NMA=4:1) solution. (a) SEM image of untreated wood. (b) SEM image of treated wood. (c) SEM-EDS image of untreated wood. (d) SEM-EDS image of treated wood. Fig. 2 NMR spectra of untreated and treated D-cellobiose. (a) 1H NMR spectrum of untreated D-cellobiose. (b) 1H NMR spectrum of treated D-cellobiose. (c) 13C NMR spectrum of untreated D-cellobiose. (d) 13C NMR spectrum of treated D-cellobiose. Fig. 3 NMR spectra of untreated and treated EHL. (a) 1H NMR spectrum of untreated EHL. (b) 1H NMR spectrum of treated EHL. (c) 13C NMR spectrum of untreated EHL. (d) 13C NMR spectrum of treated EHL. Fig. 4 Comparison of water resistance of polymer films prepared with different NMA dosage. (“V” means volume) Fig. 5 Physical properties of the treated wood. (a) Mass loss of untreated and treated wood as a function of water soaking cycle; (b) Water uptake of untreated and treated wood as a function of water soaking cycle; (c) ASE of treated wood as a function of water soaking cycle. Fig. 6 MOR and MOE of wood treated with different addition of NMA. Fig. 7 Schematic illustration and reaction route of water soluble vinyl monomers grafting into the cell walls (ML: middle lamella, CC: cell corner, CW: cell wall). 29



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Tables Table 1 Parameters of treatments for each wood samples Treatment Labels HEMA (%)

NMA (%)

CaCl2/H2O2 (%)

Untreated

0

0

0

0% NMA

100

0

2

5% NMA

95

5

2

10% NMA

90

10

2

20% NMA

80

20

2

30% NMA

70

30

2

40% NMA

60

40

2

50% NMA

50

50

2

30


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Table 2 Effect of NMA dosage on the physical properties of polymer films NMA addition (%)

Water absorption (%)

Swelling ratio (%)

0

41.68±0.83

182.25±4.72

5

38.31±0.61

139.75±6.13

10

19.71±0.68

115.75±4.92

20

8.22±0.49

104.25±2.22

30

6.15±0.27

103.25±1.71

40

4.25±0.43

102.25±0.96

50

5.28±0.39

102.50±1.29

31



Figures

Fig. 1

32


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Fig. 2

33



Fig. 3

34


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Fig. 4

35



Fig. 5

36


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Fig. 6

37



Fig. 7

38


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Table of contents

Synopsis: An environmentally friendly wood modification method was established through the reinforcement of the cell walls by the polymerization of water-soluble vinyl monomers and their reaction with cell wall components.

39


Improvement of the Performance of Plantation Wood by Grafting Water

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