Various polymeric monomers derived from renewable rosin for the modification of fast-growing poplar wood

Composites Part B 174 (2019) 106902

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Various polymeric monomers derived from renewable rosin for the modification of fast-growing poplar wood Youming Dong a, Wei Zhang b, MarK Hughes c, Miao Wu d, Shifeng Zhang b, *, Jianzhang Li b, ** a

College of Materials Science and Engineering, Nanjing Forestry University, Nanjing, 210037, China MOE Key Laboratory of Wooden Material Science and Application, Beijing Forestry University, Beijing, 100083, China c Wood Material Technology, Department of Bioproducts and Biosystems, Aalto University, Espoo, 02150, Finland d National Engineering Laboratory for Pulp and Paper, China National Pulp and Paper Research Institute Co. Ltd, Beijing, 100102, China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Wood Resins Physical properties Electron microscopy

The incorporation of polymers derived from renewable resources is more desirable in the preparation of wood composites due to its sustainability. As biomass feedstock, rosin has been used for the preparation of wood-based composite material. However, the high leachability and non-reactive with wood limit its application. Three kinds of rosin derivatives synthesized with reactive groups in the current study were confirmed by 13C NMR and Fourier transform infrared spectroscopy. The polymerization process and polymer properties of the derivatives were studied by the differential scanning calorimetry and gel permeation chromatography. The poplar wood was impregnated by rosin derivatives solution and then oven heated to induce in situ polymerization. The results indicated that the derivatives could penetrate into wood structures, polymerize in both wood cell walls and lumina, and efficiently improve the physical properties and surface hardness of wood. Moreover, the extraction indicated that rosin derivatives could permanently bulk wood structure through the formation of polymers. Our studies provide a new approach to modify fast-growing wood using renewable resources.

1. Introduction As one of the important natural resources, wood is an essential building and engineering material due to its wide availability, ease of processing, good mechanical properties, and pleasant aesthetic appearance [1]. Owing to the capability of carbon dioxide storage, wood and wood processing contribute greatly to the environment protection. The sustainability of wood and wood-based materials also accelerates its application in green composites [2–4]. However, the utilization of wood has been restricted by its dimensional instability in response to the humid environment, susceptibility to biodegradation, and changes in appearance when exposed to serious weathering [5,6]. One way to address the aforementioned drawback is wood modifi­ cation, which reduces the water uptake of cell walls through sealing or decreasing the number of available hydroxyl groups. For example, acetylation efficiently decreased the number of hydroxyl groups through the formation of ester bonds between the chemicals and the wood components [7]. Together with bulking the structure, acetylation en­ hances both the dimensional stability and resistance to fungal decay [8,

9]. On the other hand, some monomer or resin with the low molecular weight can penetrate into wood cell walls, and are then permanently accommodated in the cell walls through in-situ polymerization. This approach can also hinder the water sorption by swelling and blocking space in cell walls, such as resin impregnation, paraffin impregnation, and polymerization of styrene or methyl methacrylate [10–13]. How­ ever, most wood modification methods utilize fossil-based chemicals and these modifying agents sometimes are detrimental or harmful to the environment during its service or at the end of the product life cycle, thus not green composites. Moreover, the poor compatibility between wood components and modifying agents reduces the modification effi­ ciency and therefore the performance of resultant products [14]. Recently, much attention has been gained in using some reactive monomers and polymeric materials derived from renewable resources, such as furfuryl alcohol [15], poly (ε-caprolactone) [16], poly (ethylene glycol) [17], poly (lactic acid) [18], and vegetable oils [19]. However, the modification efficiency, product costs and up-scaling still limit the application of these new proposed methods [20]. Previously, inspired from pine resin, we explored a new wood modification method using

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Zhang), [email protected] (J. Li). https://doi.org/10.1016/j.compositesb.2019.106902 Received 11 October 2018; Received in revised form 26 April 2019; Accepted 16 May 2019 Available online 17 May 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.

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leaching, the chemical reaction between rosin and wood components or formation of polymers in wood structure is desired. Although previous studies investigated the reactive of rosin or rosin derivatives with cel­ lulose [24,25], these processes are not the same for solid wood treatment due to the special manipulating parameter and equipments. Wood polymer composite (WPC) is a good template for locking the polymeric monomers in the wood structure [13,26]. One possible approach for rosin impregnation is to anchor polymer chains in the rosin molecular structure, which modifies the reactive site, the carboxyl group, with polymerizable molecules. In this study, several polymeric rosin-based derivatives were synthesized by chemical treatments, which includes two polymerizable derivatives with double bond, rosin (2-acryloxyl ethyl) ester (R1) and rosin (2-methacryloyloxyisopropanol) ester (R2), and one derivative with epoxy group, rosin glycidyl ester (R3). The suitable chain length could help to reduce the steric effect of abietic group and remain good permeability. All derivatives were dis­ solved into 30% concentration solutions and impregnated into poplar wood followed by chemical reaction conditionally. The chemical structure and reaction activity of the rosin-based derivatives were investigated. The microstructure, dimensional stability, water resistance and hardness of modified wood were also evaluated.

Fig. 1. ATR-FTIR spectra of pristine rosin, R1, R2 and R3.

rosin as an inexpensive and renewable modifier. Our results revealed that rosin was compatible with wood, could be evenly distributed in the wood structure, and endowed wood with excellent physical and me­ chanical properties [21]. As a natural resource extracted from pine trees, rosin is abundant, biodegradable and compatible with wood [22]. The major component of rosin is abietic acid, a partially unsaturated com­ pound with three fused six-membered rings and one carboxyl group, which provides it with good hydrophobic properties [23]. Nevertheless, it is difficult to form covalent bonds between rosin and wood or form polymers through the rosin self-polymerization. Therefore, the impregnation of rosin into wood could only be attributed to physical incorporation. Hence, it is easy to be leached by solvents, consequently resulting in a reduction of efficiency. To decrease its susceptibility to

2. Material and methods 2.1. Materials The sapwood of fast-growing poplar (Populus tomentosa Carr.) was purchased from a sawmill in Beijing, China. All the samples were welldefined tangential, radial and longitudinal sections. The size of sam­ ples was 20 � 20 � 10 (longitudinal) mm3. Before treatment, all samples were extracted by solvent mixture of toluene/ethanol/acetone (4:1:1 v/ v) for 12 h in a Soxhlet apparatus to remove the extractives, oven-dried at 103 � 2 � C to a constant weight, and then stored in a desiccator. Rosin composed of rosin acids (ca. � 95%) was purchased from Shenzhen Jitian Chemical Co., Ltd. 2-hydroxyethyl acrylate (2-HEA),

Scheme 1. Synthesis routes of rosin-based derivatives: R1 and R2 contain C¼C, while R3 contains epoxy group. 2

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glycidyl methacrylate (GMA), epichlorohydrin, triethylamine (TEA), hydroquinone, dichloromethane, benzyltriethylammonium chloride, boron trifluoride diethyl etherate and 2,2-azobisisobutyronitrile (AIBN) were purchased from Tianjin Heowns Biochemical Technology Co., Ltd. Dry pyridine, sodium hydroxide (NaOH), calcium oxide (CaO), anhy­ drous magnesium sulfate (MgSO4) and other reagents were obtained

Fig. 2.

13

from Shanghai Macklin Biochemical Co., Ltd. All chemicals were used without further purification. 2.2. Procedure for the synthesis of rosin (2-acryloxyl ethyl) ester (R1) Rosin (50 g) was dissolved in dichloromethane (200 g) in a 500 mL

C NMR spectra of rosin, R1, R2 and R3. The signals of some rosin isomers were also contained. 3

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round bottom flask. Then oxalyl chloride (30 g) was added drop-wise and the reaction was performed at 25 � C for 3 h. After that, 2-HEA (20 g), TEA (17.2 g), and hydroquinone (0.03 g) were added into the system. The mixture was stirred at 50 � C for 5 h under the protection of nitrogen. After cooled to room temperature, the precipitate was removed via filtration, and the filtrate was washed with water three times. The dichloromethane layer was then dried with anhydrous MgSO4 and concentrated in a rotary evaporator under reduced pressure. Finally, R1 was obtained and used without further purification.

Table 1 Properties of the polymerization of R1 and R2 and epoxy value of R3. Sample

Mn (g/mol)

Mw (g/mol)

Mw/Mn

Epoxy value (mol/100 g)

R1 R2 R3

37588 45058 –

55673 47500 –

1.48 1.05 –

– – 0.162

12 h in a Soxhlet apparatus, then oven-dried at 103 � 2 � C to a constant weight.

2.3. Procedure for the synthesis of rosin (2-methacryloyloxyisopropanol) ester (R2)

2.6. Characterization All the obtained rosin-based derivatives were characterized by the C NMR, X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). ATRFTIR spectra were recorded on an infrared spectrophotometer (Nicolet 6700, USA) equipped with an ATR accessory; the 13C NMR spectra were obtained on Bruker Advance 500 spectrometer operating at 500 MHz with deuterated acetone as the solvent. Chemical shifts are reported relative to deuterated acetone (δ 29.84) for 13C NMR. The XPS spectra were carried out on an ESCALAB 250 XI system with Al Kα (1486.6 eV) as the X-ray source. Polymerization reactions of R1 and R2 (containing 1% AIBN by weight) were performed in an oven at 120 � C for 12 h. Then the molecular weight and molecular weight distribution of polymers were determined by gel permeation chromatography (GPC, Waters 1515, USA). The epoxy value of R3 was determined according to hy­ drochloric acid-acetone method in accordance with GB/T 1678–2008. Differential scanning calorimetry (DSC) measurements were per­ formed using a Mettler DSC apparatus (Switzerland). Polymerization reactions of R1 and R2 (containing 1% AIBN by weight) were performed directly in DSC pans under a dynamic temperature program at a heating rate of 5 � C/min from room temperature to 180 � C. The morphology of wood samples was characterized by a SU8010 field-emission scanning electron microscopy (FE-SEM, Hitachi, Japan) with a beam accelerating voltage of 3 kV. The analysis of the wood sections (approximately 20 μm) was con­ ducted using a Leica TCS SP8 confocal scanning laser microscopy (CLSM, Germany) with a 20 � /0.75 dry objective lens. The excitation wavelength was 580 nm and emissions between 435 and 500 nm were collected.

Rosin (50 g) was dissolved in toluene (100 mL) in a 500 mL round bottom flask. Then GMA (23.54 g), TEA (1 g), and hydroquinone (0.12 g) were added into the system. The mixture was stirred at 120 � C for 7 h under the protection of nitrogen. After cooled to room temper­ ature. The precipitate was removed via filtration, and the filtrate was diluted washed with water three times. The solvent layer was then dried with anhydrous MgSO4 and concentrated in a rotary evaporator under a reduced pressure. After that, R2 was obtained and used without further purification.

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2.4. Procedure for the synthesis of rosin glycidyl ester (R3) Preparation of R3 was carried out according to the previous report [27]. Rosin (50 g), epichlorohydrin (157.3 g), and benzyl­ triethylammonium chloride (0.381 g) were placed into 500 mL flask equipped with a stirrer, a thermometer, and a condenser pipe. The mixture was hold at 117 � C for 2 h and then cooled to 60 � C under the protection of nitrogen. NaOH (6.62 g) and CaO (9.272 g) were added to the flask, and the mixture was kept at 60 � C for another 3 h. After that, the precipitate was removed via filtration, and the filtrate was distilled under vacuum at 100 � C to remove excess epichlorohydrin. Finally, brown and viscous R3 was obtained and used without further purification. 2.5. Impregnation of poplar wood with R1-3 R1 and R2 were dissolved in dry pyridine to 30% concentration with 1% AIBN, whereas R3 was dissolved in dry pyridine to 30% concen­ tration with 1% boron trifluoride diethyl etherate. The poplar wood samples were immersed into as-prepared solutions under vacuum (0.09 MPa) for 30 min, then soaked under the atmospheric pressure for 2 h. After that, the impregnated wood samples were cured in an oven at 120 � C for 12 h. All resultant samples were extracted with ethanol for

2.7. Properties testing The weight percent gain (WPG) was calculated as follows:

Fig. 3. Investigation of chemical reactivities of R1 and R2 by DSC at 5 � C/min.

Fig. 4. ATR-FTIR spectra of reference and treated wood with R1-3. 4

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Fig. 5. C1s XPS spectra of reference and treated wood samples. Four peaks in de-convoluted high-resolution XPS spectrum of the C1s peaks were expressed as C1–C4 that correspond to C–C and/or C–H, C–O, C¼O or/and O–C–O and O¼C–O, respectively.

WPG ð%Þ ¼ ðW1

W0 Þ=W0 � 100

are shown in Fig. 1. The remarkable shift from 1689 cm 1 (C¼O in carboxyl group) to 1724 cm 1 (C¼O in ester bond) exhibited in all de­ rivatives, indicating that the carboxyl groups in rosin reacted with modifiers to form ester bond [28,29]. In addition, new peaks can be easily visualized in derivatives. The bonds at 1637 cm 1 and 980-942 cm 1 in spectra of R1 and R2 were related to the C¼C [30], which also indicated that the esterification reaction successfully occurred and polymerizable C¼C was incorporated into rosin. For R3, a new band at 908 cm 1 was associated with the C–O of the epoxide rings [31], which provided an evidence for the reaction of rosin with epichlorohydrin. The chemical structures of rosin and rosin-based derivatives were confirmed by 13C NMR spectra in acetone-d6 (Fig. 2). Rosin used in this work is a mixture of isomers. Except for the main component, there were some signals from the isomers for all spectra. For the spectrum of rosin, the observed peaks at 120 and 123 ppm correspond to the C¼C of rosin, and the characteristic peak at 180 ppm belongs to the carbonyl carbon of carboxylic acid. In the spectra of all derivatives, the disappearance of the signal at 180 ppm and the presence of a new peak at 178 ppm demon­ strated the ester bond had been formed [32]. Consequently, the intro­ duction of C¼C group could be confirmed by the appearance of new

(1)

where W0 and W1 are the oven-dried weights of a sample before and after the treatment, respectively. The dimensional stability was evaluated by the measurement of antiswelling efficiency (ASE) as follows: ASE ð%Þ ¼ ðSu

St Þ=Su � 100

(2)

where Su is the volumetric swelling of reference sample and St is that of treated sample. Water uptake (WU) after different time intervals of deionized water immersion and were calculated as follows: WU ð%Þ ¼ ðW2

W1 Þ=W1 � 100

(3)

where W2 is the weight of wood sample after water immersion. Contact angle analysis was conducted by a OCA 20 Dataphysics de­ vice. The tangential section of wood samples was smoothed with a sledge microtome before measurement. Distilled water droplets of 3 μL were placed onto the wood surface and images were taken at a certain frequency. Surface hardness was measured with a TH210 durometer and expressed as Shore D hardness according to ASTM D2240 (2010). Each type of sample was measured by 15 tests.

Table 2 Relative atomic compositions of carbon with different oxidation levels for reference and treated wood sample.

3. Results and discussion

Sample

The synthesis routes of rosin-based derivatives are shown in Scheme 1. Theoretically, R1 and R2 that possessed C¼C could polymerize when the initiator was present, whereas the epoxy group in R3 could only selfpolymerize and react with wood components conditionally. The ATR-FTIR spectra of pristine rosin and rosin-based derivatives

Reference R1-treated R2-treated R3-treated

5

Relative content (%) C1

C2

C3

C4

35.07 75.83 64.56 71.04

52.56 19.43 28.51 24.15

11.95 0.22 0.18 0.77

0.42 4.52 6.75 4.03

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Fig. 6. FE-SEM images of vertical sections for reference (a) and wood sample treated with R1 (b), R2 (c) and R3 (d).

peaks in spectrum of R1 (128 and 131 ppm) and R2 (126 and 136 ppm). The multiple peaks between 44 and 52 ppm in spectrum of R3 were attributed to methylene and methine groups of the epoxide rings [28], which was the evidence of incorporation of epoxide group in R3. The thermally induced free radical polymerization of R1 and R2 were investigated by DSC (Fig. 3). The strong exothermic peaks at 117.6 � C of R1 and 114.2 � C of R2 were observed, which related to the radical polymerization of the monomers. Therefore, we chose 120 � C as the polymerization temperature of R1 and R2 and used AIBN as the initiator. GPC results of R1 and R2 were shown in Table 1. The high molecular

weight of polymers indicated that R1 and R2 possessed excellent poly­ merization activity, which was consistent with the previous study [33]. However, the polydispersity (Mw/Mn) value of R2 polymer was lower than R1. The epoxy value of R3 was 0.162 mol/100 g (Table 1), sug­ gesting the presence of epoxy groups and high reaction activity [34]. The chemical changes of wood samples before and after the modi­ fication were characterized by ATR-FTIR, as shown in Fig. 4. In com­ parison to the reference, a clear shift from 1736 cm 1 to 1726 cm 1 in the spectra of treated wood samples was due to the incorporation of ester group. Moreover, the obvious increment in the transmittance of this

Fig. 7. FE-SEM images (a–d) of cross sections and CLSM images (e–h) from reference (a, e), R1-treated (b, f), R2-treated (c, g) and R3-treated (d, h) wood samples. 6

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band was observed in treated wood samples. Another change was the increment in transmittance at 2926 cm 1 assigned to the methylene link because of the introduction of carbon chain [35]. There was no char­ acteristic peak of epoxide group at 908 cm 1 in the spectrum of R3-treated sample, which can be an indirectly evidence for the ring open reaction of epoxide group. This finding was consistent with the NMR result and indicated that R3 could react itself or with wood component. The chemical changes of treated wood samples were further confirmed by the XPS spectra (Fig. 5). Four peaks in de-convoluted highresolution XPS spectrum of the C1s peaks were expressed as C1–C4 that corresponded to C–C and/or C–H, C–O, C¼O or/and O–C–O and O¼C–O, respectively [36–39]. The relative atomic compositions of carbon were calculated and presented in Table 2. In comparison to the reference, all treated samples exhibited obviously higher relative in­ tensity of C1 and C4, indicating that more C–C and/or C–H and O¼C–O were incorporated into wood samples. However, the relative intensity of C2 and C3 was remarkably decreased after treatments. Especially, the relative intensity of C2 in R2-and R3-treated wood samples was higher than that in R1-treated wood sample. This phenomenon could be resulted from the hydroxyl group that in R2 and generated from the ring open reaction of R3. Fig. 6 displayed the FE-SEM images of vertical sections for reference and treated wood samples. Compared to the reference, obvious sub­ stances were adhered to the vertical structure of all treated wood sam­ ples. The pits of R2 sample were filled with polymers (arrow in Fig. 6b). This filling could hinder the water transportation in wood structure, so that the water-induced dimensional change of wood could be decreased. The morphology of polymers in wood structure was different. In R2treated wood sample, the polymer exhibited a bulk type with brittle rupture surface (arrow in Fig. 6c). And the polymers in R3-reated wood

sample were dispersedly adhered to the wood cell lumen wall (arrow in Fig. 6d). For the cross section, the natural cellular structure of poplar wood with irregular cell shapes can be observed from the reference sample (Fig. 7a). After the treatment, part of wood cells with filled lumina can be viewed in all treated samples (Fig. 7b–d). These results suggested that the wood structure could be permanently filled with rosin derivatives through the chemical reaction. Moreover, the presence of polymer in the cell lumen of R3-treated wood sample indicated that selfpolymerization of R3 could occur. Another important finding was that the polymers from R2 exhibited slight cracks between polymer and wood cell lumen wall when compared to that of R1 and R3. This phe­ nomenon could be due to the different morphology of polymers. This weak adhesion could lead to the limited improvement in dimensional stability. To better detect the distribution of modifier, the fluorescence signal of rosin can be tracked using CLSM. As shown in Fig. 7e, rather weak fluorescence can be seen from the reference wood when exciting at a wavelength of 580 nm. However, a strong fluorescence emission was observed from the treated wood samples (Fig. 7f–h). For R1- and R2treated wood samples, the strong signal can be observed in both the cell walls and the filled cell lumina, suggesting that the rosin compo­ nents penetrated into wood cell wall, polymerized in both cell walls and lumina, and then further swelled the wood structure. This distribution characteristic was also observed in rosin impregnated wood samples [21]. However, only cell wall fluoresced in the R3-trearted wood sam­ ples (Fig. 7h). Actually, most cell lumina were filled with polymers in R3-treated wood sample, which can be seen from Fig. 7d. It is possible that the opacity of filled cell lumen in wood slice where the polymer density was higher than that in cell wall. Because the cell wall restricted polymers formation [40], the molecular weight of polymer located in

Fig. 8. Physical properties of the modified wood. (a) Wetting behaviour of reference and treated wood samples. (b) Water uptake capacity of wood samples. (c) WPG and ASE of the treated wood samples. (d) Surface hardness of reference sample and treated wood samples. 7

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cell lumen could be higher than that in cell wall. After the treatment, the wettability of wood surface was changed (Fig. 8a). Compared to the reference, treated wood samples showed higher contact angle at different contact times. Moreover, the water uptake of all treated wood samples was significantly reduced compared with the reference (Fig. 8b). This result could be related to the filling effect and high hydrophobic characteristics of rosin-based polymers. The effects of three modifications on the filling could be different. The R3-treated wood had the lowest water uptake while the highest contact angle, suggesting the penetrability of treated wood was lower than others. The impregnated pristine rosin was easily leached because of the absence of chemical reactions. However, the average WPG of treated wood samples after extraction was still about 25%–35% (Fig. 8c), implying the permanent filling of rosin derivatives. This higher polymer loading indicated the reaction of R1-3 occurred in wood structure, which accordingly resulted in an efficient ASE. The average value of ASE for the R1-treated wood sample was near to 50%, which was higher than the ASE of rosin-impregnated wood that was 36% when WPG was about 32%. This increment could be due to the bulk of R1 polymerization. Notably, the ASE value of the R2-treated wood sample was lower than that of R1-treated wood, although they had the same reaction mecha­ nism. It is possible that the higher molecular weight of R2 restricted the penetration into wood cell walls. All treated wood samples showed higher ASE than that of wood filled with vinyl monomers, such as sty­ rene (ST) and methyl methacrylate (MMA) [20,41]. The main reason could be the excellent compatibility of rosin components, thus polymers can bulk the micro- and nano-voids in the cell walls (Fig. 7f–h), while the polymers of ST or MMA mainly filled in the cell lumina [14]. Therefore, the present study indicated this approach could increase the resistance of leachability of rosin and endow wood with excellent physical properties. The surface hardness of wood samples was also enhanced by this insertion of rosin derivatives. As shown in Fig. 8d, the surface hardness of R1-, R2-and R3-treated wood samples were increased by 18.6%, 20.9% and 24.1%, respectively, compared with the reference.

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4. Conclusions The rosin-based derivatives were successfully synthesized by various synthetic strategies, which were confirmed by ATR-FTIR, 13C NMR, and DSC. The derivatives can penetrate into poplar wood, polymerize in both cell wall and cell lumen, and swell wood structure, resulting in improved dimensional stability, low wettability and water uptake, and high sur­ face hardness of wood. Moreover, due to the polymerization of rosin derivatives in wood structure, the susceptibility to leaching was signif­ icantly decreased when compared to the pristine rosin impregnation. This approach can provide a new method to fully use the renewable resources for preparation of high quality wood composites. Conflicts of interest The authors declare that they do not have any conflict of interest. Acknowledgements This work was the National Natural Science Foundation of China (51779005/E090301), the Fundamental Research Funds for the Central Universities (NO. 2016ZCQ01) and the Scientific Research Foundation for Talented Scholars (163020124). Appendix A. Supplementary data Supplementary data related to this article can be found at https://do i.org/10.1016/j.compositesb.2019.106902. 8

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